Chess960 (Fischer random) is a chess variant created by Bobby Fischer where the pieces on the back rank are randomized, there are 960 possible starting possitions, hence the name Chess960
As you can see the pawns a still on rank 2/7 but the pieces on the back rank are randomized, there are a few rules to how the pieces are randomized.
- Bishops must be on oposite colours
Rooks must be on either side of the king, this means the king can't be on the a/h files
Castling: To castle in Chess960 is the same as in normal chess,in kingside casling the king is put on the g-file and the rook is put on the f-file and in queenside castling the king is put on the c-file and the rook is put on the d-file. In Chess960 if the king move it cannot castle and if a rook moves the king can not castle that way (just as in normal chess). To castle in Chess960 online you must put the king on top of the rook.