There are three styles of contracts that you typically will run across. Some modeling agencies will work with exclusive contracts. This means that they are your exclusive representatives and they get a commission on any work you do. They get this commission whether they find the work or you find it on your own. If you sign this kind of contract, be sure the agency is really going to be working for you.
If they are providing a lot of guidance getting your career started and are out there beating the bushes to find work for you, this type of contract is fine. If the agency signs you just to fill space in their roster and then leaves you hanging in limbo for the length of your contract (usually one to two years) it can be a bad way to try and start a career. Another type of contract is a non - exclusive modeling contract. In this case if the modeling agency finds you work, they get paid their commission.
If you find work on your own you pay them nothing. You are free to sign non - exclusive contracts with other modeling agencies. This way you might have several agencies representing you at once. Now don't expect the same service and help from a non - exclusive agency as with an exclusive agency. In smaller markets, where you are expected to provide all of your training, portfolios, composites and other marketing materials before an agency will work with you, signing an exclusive contract is a kind of a rip - off.
The final contract is a Mother Agency Agreement. i have not seen one of these so i am not sure how they make it legally binding but you do hear about them. In this case the modeling agency knows there is not enough work locally to support any one over the long term.
Their position then is to move you on to a major market and get you signed with a major modeling agency. They may get you work locally but it is more with the idea of getting you ready for the major market.
For helping prepare you and helping to place you with a top modeling agency, they get a percent of your earnings for a long time to come. You, in essence, never leave the mother agency - you are just loaned out to the top agencies. This can be an attractive arrangement if the mother agency invests time and money in your career and gets you on with top money making agencies.
The trouble comes when this gets attached to a modeling school. You not only have to pay for all of the classes, the photo shoots, the career guidance session, you then must keep paying these people again for doing nothing. You need to examine these agreements very carefully before you sign.