What is a trill on the guitar?

A trill is a rapid movement between two notes that can be characterized as a decoration to make a melodic line more interesting. The two notes can be played a whole step or half step apart. On the guitar, trills are most often played by a hammer-on and then an immediate pull-off. Conversely, the guitar can also perform a trill when the performer rapidly alternates between notes on adjacent strings. This technique is called a cross-string trill.