A mandolin is a musical instrument considered to be part of the lute family. Mandolins are tear-shaped and have eight strings in pairs. Mandolins can be tuned in a variety of ways. The most common is the violin tuning, GDAE, with each pair of strings tuned in unison. Mandolin often can be heard in country and bluegrass music, but is used in rock as well--of particular note by Rod Stewart in several of his hits, including Maggie May.
Chop Strumming
The chop strumming technique is one of the fundamental strumming patterns in bluegrass music. The chop is played sharply with alternating up and down strokes, with the first strike coming on the first and third beats. It has a very rhythmic feel and plays a time-keeping role in the music. Typically you will mute the strings with your strumming hand on the down strokes and let let the strings ring on the up strokes to produce a percussive sound. You can recognize the chop method of strumming by the strict downward, almost chopping motion, of the strum hand on the down stroke.
Folk Strumming
Folk strumming is a key strumming pattern used in bluegrass and country. Emphasize the first beat of every bar of music with this pattern by dragging your pick across all the mandolin's strings, from the thick strings to the thin strings, then fill in the rest of the bar with quick, alternating up and down strums. Let the first beat strum ring as you play the quicker strumming fills. Add some string muting with the side of your hand to break up the the rhythm.
Ringing Pick Strum
The ringing pick string is heard frequently in folk-type campfire mandolin playing. There is no set rhythmic structure for this strum. Rather, it involves strumming the chords in alternating up and down strokes while letting them ring fully. While strumming in this fashion, randomly strike only the lower or upper pairs of strings to add tonal color to the song. Do it by feel and what sounds good to you. There are no rules about how many times to add just the upper and lower pairs. Practice will train your ear for this type of strumming.