1) SAS is an integrated developemnt / analysis environment, which includes a number fo package, such s SAS/BASE, SAS/Stat, SAS/Connection, SAS/IntraNet, etc. The Most popular one is SAS/BASE. You can think it's new development language (so-called 4th generation language). Generally speaking, it's a very powerful software package.
2) SAS can be running on various platforms, like OS390, AS/400, UNIX, NT, and other. It can also be interfaced with almost all DBMS, like Oracle, SYbase, DB2, VSAM, even Excel, Access.
3) I have been using SAS on OS390, UNIX, and Windows NT. I think it's not hard to learn but you need to spend time in order to master it.