Guild Dues[]
The most obvious responsibility is that the guild member must pay dues to the guild. These will be variable. One guild may include training costs in the basic dues, which will then be fairly high; another guild might not, charging for training as and when the guild member needs it, so their basic dues will be that much lower. Of course, if the DM doesn't use training rules in his game then this element will be absent. When the DM comes to design a thieves' guild using the system outlined later, this is a important element to consider!
In addition to a basic membership due, the guild may insist on taking a certain percentage cut of the rewards the guild member gains from his thieving activities. Here, it is likely that there will be a "basic rate" for normal operations, with guild seniors considering special cases individually. For example, if the guild provided the thief with certain important information which made a big difference (and indeed without which a robbery would have been unsuccessful), the guild may take a larger slice of the goodies than normal. What cut the guild takes will again be variable. Some guilds may discount the yearly dues (possibly in part) from later cuts from income.
Secrecy[]
This is just as important as paying dues. The guild member will be expected to keep the identity of the guildmaster (if he knows it), the guildhouse, and planned guild activities secret from outsiders. He must never inform on a guild member.
In a guild dominated by a lawfully-aligned guildmaster or group, secrecy may be the most important of all commitments the guildmember has to make. Some form of oath of loyalty is very likely to be demanded of the novice in almost any thieves' guild.
Providing Information[]
There are two quite specific ways in which a thief will be expected to supply his seniors within the guild with information. First, information concerning his own plans; second, general information possibly of value to guild members.
A thief will be expected to keep the guild informed about important ventures he has planned. This does not include dungeon-bashing, wilderness adventures, and the like! Rather, robberies and break-ins which may net particularly good rewards or strike at "sensitive" targets (such as a temple, or the warehouse of a notable merchant), particularly nefarious crimes such as kidnapping, and major scams are examples of crimes which a thief will always be expected to notify the guild seniors or guildmaster of. Many guilds will insist that such major crimes can only be committed with the guildmaster's permission. This is especially likely to be true with a strong guild, and in a lawful or repressive society where such actions may stir up a hornet's nest of trouble for the guild.
Feeding back general information to the guild will also keep a thief in good standing with his fellows. A thief casing a joint for a later break-in might see an unusual number of guards around in the courtyard of an adjacent building as some crates are being taken inside from a wagon, and one crate splits slightly at the top revealing the glint of gold or ivory in the bright sun... The thief knows that he cannot organize a robbery over there as well as in the place he's casing already, so he reports the pleasant view to the guild. Doing so will certainly keep the thief in good favor. The thief will be expected to provide such information should he come across it, but the guildmaster might well reward particularly helpful guild members, mark them for promotion, give them a sinecure role in some other guild job, pay a small percentage of the guild take to the helpful informant, and the like.
Snitches: These can be of special importance to a thieves' guild. Basically, a snitch is someone paid to provide "inside information." A snitch can be a corrupt officer of the law, a valet or servant of a rich man (or at a club of some kind), a nightwatchman, and so on.
A few guilds might insist that only the very senior guild members can have their own snitches; juniors must turn over promising contacts who could be developed into snitches to the guild. More likely, though, is that individual members can have their own snitches who are regarded as sacrosanct by other guild members. A guildsman does not tamper with another thief's pet snitch! This is obviously going to be easiest if the identities of the snitches are actually commonly known within the guild—how can you not poach a snitch if you don't know who is one, after all? But, because some thieves may be very nervous about this, the identity of certain really important snitches may be kept secret. The guild will protect them by warning thieves away from the key snitches indirectly (e.g., with an edict that no one is to try to "make friends" with the staff of such-and-such an establishment). This does not make the identity of the snitch known, but serves to warn other thieves away.
Limited Territory[]
The thief will almost certainly be told that certain areas and activities are definitely off-limits. This is likeliest to apply to major crimes and big heists when junior thieves are the hopeful plotters (the guild is unlikely to allow such inexperienced people the chance to bring the wrath of the law down on everyone's head). But territorial restrictions may be just as important.
The simplest form of this is that certain thieves will have their "own patch". Pickpockets are the most clear-cut example. A notably busy thoroughfare, one where merchants and (especially) foreigners throng, is a patch which a skilled group of pickpockets will fight determinedly to keep as their own, exclusive territory. Protection rackets are another obvious case of a demarcated territory where other guild members do not stick their noses in. These will include warehouses and offices and homes which are off-limits to burglars, because their owners pay a sum to the guild to avoid being robbed.
A more complex example of this is where sub-guilds control definite sections of a city and expect that only their own people are usually allowed any activity at all within that section. Exceptions are allowed only after careful consideration by the leader(s) of this group. This situation may happen if a guildmaster is weak and the second-rankers start carving out territory for themselves, but it might arise for simple reasons of historical accident (in a walled city with major internal divisions, gates between town quarters, and so on—the City of Greyhawk is an example). At its most extreme, a city might in effect (if not in name) have several thieves' guilds, each controlling one section or quarter of the city, with the boss of each splinter faction meeting with the others at regular times to try to co-ordinate efforts and defuse tensions.
Other Duties[]
Depending on the individual guild, the thief may have other responsibilities. These can vary considerably, but some of the more common cases are detailed here.
Training: The thief may only be allowed to train apprentices of his own with the knowledge and permission of the guild, and the guild will probably want a cut of the training fee. Some guilds will regulate the allocating of apprentices to trainers. In busy times when the guild is expanding, middling-level thieves may have to give up a certain amount of time, demanded by the guild, to train new apprentices.
Legwork: Especially with apprentices and low-level thieves, the guild may require them to do some of the tedious day-to-day legwork, at least as backup reserves. This can involve collecting money from the victims of protection rackets, collecting and carrying bags of small change from illegal street gambling, acting as a messenger boy, keeping a part-watch on a warehouse, and so on. This shouldn't be too oppressive, but a sudden need for extra help on the part of the guild is a nice plot device for keeping a thief PC in town and maybe having him see things, and meet people, that widen the scope of his future adventuring.
The Recalcitrant PC Thief[]
A player may decide that he doesn't want to have his thief PC join any guild; this may be especially likely with Chaotic characters (and Chaotic players!). It's best not to pressure the player in this case, but as his thief PC gains experience he will inevitably tread on the toes of the thieves' guild (e.g., by robbing someone they have an "understanding" with). The guild will sooner or later have to take action against the PC thief. They will warn him first, and stress the benefits of joining (as explained above). Eventually he will have to join up, or ship out. There is also the matter of training, of course; if he doesn't get this from the guild, where else can he come by it?
Some players may worry that their thief PCs may have their freedom of action massively curtailed, or their alignment may be compromised (especially if the PC is of Good alignment), or their PC may simply be ripped off. These are legitimate worries, and some players may want to be reassured about them. The DM should be able to lure all but the most uncooperative player into guild membership without any strong-arm tactics, and then the full plethora of the guild's nefarious activities can be unfolded...