If socks are white markings on a horse's legs that are fairly short, then obviously stockings are fairly high. Sometimes the term "high white" is used - in this case generally for white going all the way up the leg, which is often indicative of sabino (If white is also on the flanks and belly, then it is definitely one of the sabino variants).
Socks stop below the knee or hock. Stockings carry on above it.
Stockings are common on pinto horses that carry the tobiano, sabino or splashed white genes, but can also be seen on otherwise solid animals.
This Paint horse has stockings as well as everything else. (Based off of the ragged patch on his shoulder, this is a tovero horse).