An ornament is a note or group of notes that decorates and gives character to a melody. The simplest of all ornaments is the grace note, a quick note written smaller than normal with a slash through its stem. Grace notes are played as fast as possible and usually slurred to the note they decorate.
A trill is an ornament of at least two alternating adjacent notes. Like the grace note, it’s written in smaller font.
The following solo from The Shearer Method, Classic Guitar Developments, Book II, Gavotte (p. 160), provides practice with both grace notes and trills.