What is the difference between a cross object formula and a roll up summary?

Ans:-
There are many but the main one ofcourse if that a cross object formula references fields of its parent records to perform various operation and return a result where as a roll up summary calulates the Count, sum,max, min of some fields in the child records. Also , a roll up summary is not actually calculated and stored in the database. It is calculated only at run time. Were as a formula is calculated and stored like any other field. There are a some more but these are the cream of the cake .