First off both teams are allowed to have 6 players on the ice at one time. most of the time teams go with 1 goalie, 2 defenseman and 3 forward.
The defenseman typically line up on either side of their own goalie on the defensive side of the ice and on the offensive end they usually line up on the opponents blue line. The defenseman's job is too protect the net in the defensive zone and man the point on the offensive zone.
The forwards typically line up in 3 positions, left wing, center and right wing. The center takes mosts of the face offs and plays the middle of the ice. The left and right wingers play along their respective sides. In the offensive zone the forwards lead the attack and try to score goals. On the defensive end the foward ususally line up around their own blue line and do come further down into their zone from time to time to help out their defenseman.
Also here's a diagram that shows all the lines and whatnot on the ice:

Also here's a little video that explains things a little better and with a little more detail:
[youtube]http://www.youtube.com/watch?v=2QKb8ngEucw[/youtube]
I hope all of that helps you out.