SRD:Prone

Prone
The character is on the ground. An attacker who is prone has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A defender who is prone gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.

Standing up is a move-equivalent action that provokes an attack of opportunity.