Although you will hear people say both "offside" and "offsides", "offside" is the proper term.
Offside can be one of the most confusing and controversial parts of the game of soccer. The causes of confusion include:
1. The key issue is where the player is at the time of the pass — not at the time the ball reaches the player’s vicinity, so it is difficult for fans and officials to focus on both areas simultaneously.
2. A player who is not actively involved in the attacking play, is not considered offside regardless of position.
For an outstanding video explanation, link to "Video Explanation" (requires Macromedia Flash Player v. 5 or later — which most people have without knowing it). (http://www.dynamic-thought.com/offsideClicketteLo.html)