A creature with the summon spell-like ability can summon specific other creatures of its kind much as though casting a summon monster spell, but it usually has only a limited chance of success (as specified in the creature's entry). Roll d%: On a failure, no creature answers the summons. Summoned creatures automatically return whence they came after 1 hour. A creature that has just been summoned cannot use its own summon ability for 1 hour.
Most creatures with the ability to summon do not use it lightly, since it leaves them beholden to the summoned creature. In general, they use it only when necessary to save their own lives.
An appropriate spell level is given for each summoning ability for purposes of Concentration checks and attempts to dispel the summoned creature. As stated on page 37 of the Dungeon Master's Guide, no experience points are awarded for summoned monsters.