as far as active/inactive go, i'm pretty sure an ACTIVE session is only one who is in the middle of doing work. executing a statement, procedure, etc.

if i log onto sqlplus and let it sit there while you query sessions, mine will be INACTIVE. if i execute a query that takes 10 seconds, and you query sometime during those 10 seconds, i will be ACTIVE.

what applications are running against your database? some applications may make persistant connections.