A cantrip is a spell of the lowest spell level (0). Cantrips are simple spells, with minor effects that do not improve as caster level increases. They are considered so simple that a wizard is allowed to cast cantrips from an opposition spell school.
Rangers and paladins do not gain level 0 spell slots, but paladins do have access to two cantrips (resistance and virtue) as level 1 spells.