In any design or development process, there is an outlined list of what is expected from the designer and developer as requested by the client called the scope of work. This document will ensure that both parties know exactly what to expect from any given client and make sure that the client gets what they hired the designer/developer to do and that the designer or developer isn’t required to do things that weren’t agreed upon in the original scope. This is not a perfect world, though, and oftentimes new requirements or features will be needed after the original scope of work has been determined; scopes of work can be amended to include the new items at which point the designer or developer will most likely increase the payment amount to reflect the added work.
Scope creep happens when a client continues to request additional work, features, or time without renegotiating the final payment amount - the scope of work creeps upwards. Most developers and designers are nice people who will probably allow one or two new items without charging more, but this courtesy can be abused by clients who feel entitled to unlimited changes. This is a bit like trying to hit a moving target as the checklist for completion keeps growing. For the most fair and square dealing with a developer or designer, any major changes in the scope of work should warrant a renegotiation and new scope lest the developer ends up effectively working for free.