Spell-like abilities (Sp) are magical and work just like spells (though they are not spells and so have no verbal component, somatic, material, focus or XP components). They go away in an antimagic field and are subject to spell resistance if the spell the ability resembles or duplicates would be subject to spell resistance.
A spell-like ability usually has a limit on how often it can be used. A spell-like ability that can be used at will has no use limit. Using a spell-like ability is a standard action unless noted otherwise, and doing so while threatened provokes attacks of opportunity. It is possible to make a Concentration check to use a spell-like ability defensively and avoid provoking an attack of opportunity, just as when casting a spell. A spell-like ability can be disrupted just as a spell can be. Spell-like abilities cannot be used to counterspell, nor can they be counterspelled.
For creatures with spell-like abilities, a designated caster level defines how difficult it is to dispel their spell-like effects and to define any level-dependent variables (such as range and duration) the abilities might have. The creature's caster level never affects which spell-like abilities the creature has; sometimes the given caster level is lower than the level a spellcasting character would need to cast the spell of the same name. If no caster level is specified, the caster level is equal to the creature's racial Hit Dice.
Some spell-like abilities duplicate spells that may work differently or be of a different level when cast by characters of different classes. A monster's spell-like abilities are presumed to be the sorcerer/wizard versions. If the spell in question is not a sorcerer/wizard spell, then default to cleric, druid, bard, paladin, and ranger, in that order.