If a players entire foot crosses the centerline of the court under the net while in the middle of a play, it is a line violation and point for the other team.
If a player touches the white tape of the net while making a play, it is a net violation and point for the other team.
*There are some exceptions to these rules. If a player is in the net or crosses the centerline, but the play of the ball is nowhere near the violation, it is waved off and not called. These rules are primarily for safety.