Complex sentence: A pajama party (also spelled pyjama party and sometimes called a slumber party) is a form of overnight party, usually for children, in which participants wear pajamas, stay up late, and talk and play until they fall asleep.
Split into simpler sentences: A pajama party (also spelled pyjama party) is a specific type with some focus on the clothing worn. The participants typically stay up late, and talk and play until they fall asleep.