New to SCID, Where should I begin? I read "SCID database" mentioned many times in different chess forums. As a regular chessbase/pgn user, I wonder what difference it makes and where I should begin. Thanks!
The Start here section of the SCID official webpage provides a lot of different links explaining the functionalities of the program, what can you do with it and what can you do to help back, among many others. 

SCID provides a free alternative to commercial software, compatible with the standard .png format but with an own format .si4 said to be one of the key points when it comes to the speed of SCID analizing and mantaining a huge database.

It is compatible with very powerful engines like Rybka or the famous Stockfish used for everyone from an amateur to top GMs.

This is just an intro to all the info that you can find in the official page. The FAQ section is also very useful if you're approaching SCID for the first time.