This article is from the Dog Supplies FAQ, by Cindy Tittle Moore with numerous contributions by others.
Again, there are many kinds of leashes, in different lengths. You will
probably want a short leash for walking in crowds, a longer leash for
just walking along, and an extra long leash (that could just be rope)
for some training exercises.
You can find a variety of snaps on leashes. The most common is a hook
with a knob that pulls down to open the hook (snap hooks). Another
kind is a hook where the lower part pushes in (spring hooks). The
latter are better as they don't accidentally release. Look for hooks
with swivels to avoid twisting. There are a few hooks that actually
screw closed. They are hard to find but can be useful for some people.
* Flat nylon leashes. The most common. They come in a variety of
colors and lengths.
* Braided rope leashes. These look like the rope used in rock
climbing, with the same colorful patterns. These are sometimes
easier on the hand and are quite sturdy.
* Leather leashes. These range from the plain to the intricately
braided. Take care to keep them out of the water to prevent
brittleness. Inspect them for wear. Shorter ones, two to four
feet, are ideal for training.
* Metal link leashes. Especially if your dog likes to chew on
leashes. Sometimes combined with leather, especially for the
handle. Not a good leash to use with a choke-chain collar.
* Flexi-leads. Developed in Germany, these are spring-loaded,
retractable leashes that have a minimum length of 2.5 feet and
varying maximum lengths. They come in a variety of sizes. The
handle is bulky because it contains the retracting assembly, but
there is a comfortable hand grip. Be careful -- it is easy to get
wrapped up in the flexi-lead and rope-burn yourself or at least
get all tangled. These leashes are ideal for letting the dog
explore around you while you walk along. They are not very good to
use when training your dog because of the amount of give in the
rope even when the length is locked in.