GUEST, APPLSYS, APPLSYSPUB, APPS users:

GUEST is like a Dummy Account. By default it has ORACLE as password.
GUEST account used by JDBC Drivers and Oracle Self Service Web Applications like istores, irecrutiment, iprocurement, ipayables, ireceivables etc to make initial Connection.

APPLSUSPUB Account is used at Initital Client Connectiviey to Oracle Applications.
The Name tells that it as a PUBLIC Account.
When User Login in to Applications using his username and password. These credentials (Username and Passwords) are validated by login into APPLSYSPUB Account. APPLSYSPUB account is Gateway User ID / GWYUID.

APPLSYS is same as other oracle users like AP, AR, GL etc whose hold their set of tables, views etc. Same way APPLSYS Account holds its set of tables like FND_USER and FND_APPLICATION, AD_APPLIED_PATCHES etc.



APPS is central Schema which holds synonyms for all other Users Database Objects.
One main thing is APPLSYS and APPS should have same password. Why these contains same password.

Apps is a universal schema has synonyms to all base product tables and sequences. This also has code objects for all products (triggers, views, packages, synonyms etc.).

Applsys schema has applications technology layer products like FND and AD etc.

Both need to have same password because when you sign on to apps, initially it connects to a public schema called APPLSYSPUB. This validates AOL name and password that we enter (operations/welcome). Once this is verified we select responsibility, this is validated by APPLSYS schema and then it connects to apps schema.

Since it uses both applsys and apps during signon process this expects both the password to be identical. Try changing apps password to something else and try to login, the validation at the last stage would fail. This would result in failure of application login