Monofilament is a nylon based fishing line that is one of the most popular lines around.  Lending to its popularity is the fact it is a relatively inexpensive and versatile line.  Monofilament has average toughness, sensitivity and visibility.  It is easy to knot to a hook or lure and is “stretchy”.  The stretch lends well to applications where you do not want to pull the hook from the mouth of a fish on a hook set or allows for some forgiveness when a fish makes a big run near the boat.  The down side of the “stretch” is when left on a spool to long it develops memory and can coil off the spool and become tangled.