a tremolo is when you use your lower arm muscles to move a string instrument's bow very fast. in my orchestra music, the marking for tremolo is 3 short, 35 degree angle dashes that sit in between lines 2-4 on the treble clef. i'm not sure if the dashes can increase or decrease whether on how much the composer wants you to do tremolo.
a trill is usually have one finger down on a string and you move the next finger up and down quickly. this will create a series of 2 notes, the top being longer than the trill by a bit. trills are used to articulate notes to make them sound less plain.