We face this question many time in OTN Discussion Forum .Mainly;

Apps is a schema which does not contain any tables of itself. We can say APPS is the shared runtime schema for all E-Business Suite products. It contains all the synonyms of all the table in Oracle apps. But it also contains packages, functions, procedures. The default password is apps.

Applsys schema contains all the tables required for administarative purpose. The default password is apps. All the technical products’ database objects are consolidated into a single schema called Applsys

Applsyspub schema is responsible for password checking.The default password is pub.Applsyspub is used for authentication by having read only views.