Castling?
REAL MEN stay in the center.
Castling is a powerful move, like 3 moves in 1: Kf2, Rf1, Kg1. It brings the king to safety and connects the rooks.
O-O is generally better than O-O-O. It is easier to achieve in 4 moves, while O-O-O needs at least 5 moves including a premature queen move. The king is safe at g1, while at c1 it is not and it often needs an additional move Kb1 to be safe and to protect pawn a2. The only exception is when you intend h2-h4 developing Rh1 on its starting square.
It's usually better to castle on the king side because it is closer to the corner. Queen side is 2 spaces away from the corner, while king side is only 1
The amount of O's in the castle notation depend on the amount of spaces the rook jumps
In the vast majority of games King-side castling is best. Of the remainder, Queen-side castling is most likely to be best. Choosing not to castle is best in the fewest matches. Unexpectedly losing the right to castle is almost always bad.
Both castling sides are safe but it really depends on the position.