Clarifications regarding logon groups

Hi All,

I need some info regarding logon groups. I know we can create and maintain logon groups using txn SMLG.

I'd like to know exactly what logon groups are?

For what purposes do we need them? i was told that logon groups are normally used when debugging applns (so as not to hinder performance). Can anybody throw more light on this? Thanks!

cheers

Prachi