Fork: A fork happens when you move your knight or one of your pieces to a square that you can attack two pieces on at the same time, so you will get one of the pieces. You can also do a fork with different pieces including the queen, bishop, rook, and knight, all of the main pieces. The bishop and the queen can do it by taking a piece and attacking the king and the rook. To prevent a fork against you, you can move a piece and guard the attacked piece.

Stalemate: A stalemate happens when a players king can't move any where without being taken by a weaker piece. So if the black player moves it's queen somewhere attacking a piece that is stuck in where it is, then the white king can't move anywhere, so it is a stalemate.

Checkmate: A checkmate happens when one of the players checks the other players king, so it has to move. But a checkmate is when it can't move anywhere else and that is what a checkmate is. 

Double Check: A double check happens when you move one of your pieces (for example your bishop and the rook is behind it),You move your bishop and check their king, but you also check it with your rook because you revealed the rook by checking him with the bishop.

Pin: A pin is something were a piece is attacking a piece, but it can't move it because there is a better piece behind it like the king or queen. You shouldn't take the pinned piece right away or you will be giving away a piece that is key to part of your attack.(For example if the knight is diagonal from the queen, if the knight moves the player will take the queen with the bishop.)

These are the main tactics for chess and this site.