Policies and agreements vary considerably between publishers, and sometimes even between journals owned or managed by the same publisher. The SHERPA/RoMEO database offers a summary of publisher copyright & self-archiving policies. Columbia Law School--Kernochan Center for Law, Media and the Arts has sample clauses found in publishing contracts taken verbatim from the contract and explained in plain English as well as a helpful rating system.
Here are some examples of the breadth of Academic Journal Publication Agreements:
However, publisher policies evolve over time and the general terms stated on their web sites can vary from the terms actually sent to authors, so it is important to read the agreement itself.