SRD:Frightened

Frightened
A frightened creature flees from the source of its fear as best it can. If unable to flee, it may fight. A frightened creature takes a –2 penalty on all attack rolls, saving throws, skill checks, and ability checks. A frightened creature can use special abilities, including spells, to flee; indeed, the creature must use such means if they are the only way to escape.

Frightened is like shaken, except that the creature must flee if possible. Panicked is a more extreme state of fear.