Leads can be used to lead mobs around, or to tie them to fenceposts.
They are crafted from string and a slimeball.
Leads work on most passive mobs - ie. all animals except bats, but not hostile mobs.
To attach the lead to a mob, just right-click. You can then either lead the animal around, or tie it to a fence-post. The lead will stretch up to 10 blocks before breaking.