Summary Account is an account whose balance represents the sum of other account balances. You can use summary accounts for faster reporting and inquiry as well as in formulas and allocations.

Rollup group is a collection of parent segment values for a given segment. You use rollup groups to define summary accounts based on parents in the group. You can use letters as well as numbers to name your rollup groups.

Summary Account is an account whose balance represents consolidation of accounts

Rollup group is a collection / consolidation of parent accounts. Eg Assets is a total of Current assets and Fixed Assets. Current assets / Fixed assets in turn are collection of assets.