What's the difference between a ristretto and an expresso?

First off, there is no such thing as 'expresso", it is eSpresso. (Sorry, personal pet peeve).

Ristretto is a way to pull a shot. You can pull the shot either 'ristretto' or 'restricted' or you can pull it 'longato' or 'long' Ristretto shots are stronger in taste because the espresso is ground finer than normal so the shot takes longer to extract. A normal shot should take between 17-22 seconds to extract. Ristretto shots will take about19-25 to extract and should have more crema (the yummy carmel-colored top of the shot) than a longato shot.
Longato shots are a little less potent as ristretto, but still quite lovely in the hands of a good barista. The grind is a little more coarse and will take about 15-20sec. to extract, the stream will be thicker, and come out a touch faster. This shot will have less crema than a ristretto, but should still be about half of the shot.
A ristretto is a an even stronger version of an espresso, much more concentrated.
expresso is a kind of Italian coffee, I don't know what is ristretto . Sorry !!
a ristretto is an espresso made with less water, which makes it more concentrated.

for a more elaborate discussion have a look at
all i know is that exspresso is very strong and tastes yuk