Trills are embellishments using two adjacent notes alternating quickly.
Practice trills keeping your fingers as close to the strings as possible. A good way to practice trills is with scales playing a trill on each note. Begin slowly and work up the speed with time. Playing trills will help increase finger dexterity and speed. Most trills are marked above a note in music and played one half or hole step above the note. In baroque style the trill usually begins on the upper note unless the previous note is the auxiliary note. More tutorials on http://fiddlerman.com