Spell-like abilities are magical abilities that work similar to spells, but are not cast from a spellbook. Instead they typically have a fixed number of uses per day. Spell-like abilities rarely cause an attack of opportunity, require a concentration check, or can be counterspelled. In addition, since these are not cast from an arcane spellbook, they are never subject to arcane spell failure. If the ability is not identical to an existing spell, it is usually applied as an extraordinary effect and is thus not subject to dispelling.
Spell-like abilities can be obtained from various sources. Perhaps the most obvious is by assigning one to a creature in the Toolset via the "Special Abilities" tab in the creature's properties window. Another common source is from certain feats, usually class feats. They can also be obtained as a result of a polymorph, or granted by a specially scripted AI. It is only for these last two sources (polymorph and scripting) that a spell-like ability might ever generate an attack of opportunity, be subject to a concentration check, or be counterspelled.
The caster level of a spell-like ability depends on how the ability was gained. Abilities granted through the Toolset have their caster level specified there, with "monster abilities" having a caster level of zero. (The caster level field is inaccessible for "monster abilities".) Abilities obtained via class feats have a caster level equal to the corresponding class level. The remaining abilities use the class level of the first class a creature has.
10 + level + ability modifier
The major difference compared to spells is that for class feats, the level in question is the corresponding class level, which can be much higher than the highest spell level. For other spell-like abilities, the level used is the innate level of the ability. The ability modifier used in this formula is that of the primary ability of the class used to cast this ability for class feats, and that of charisma in other cases.