IDing snails

What shape are they? Pond snails tend to be sort of football shaped (in different species the shell curls to the right or the left), ramshorn snails are shaped like a flat coil, and Malayan trumpet snails are shaped like a cone. Apple snails get big, and unlike the others who may hitchike in on plants, they generally have to be purchased.
Some types of pond snails will eat plants, and there are some species of apple snail that will munch on plants (my Pomacea bridgesii doesn't, maybe I'm just lucky). You can try google as well - there's a surprising amount of information out there about aquarium and pond snails.

Actually pond snails are the flat coil. Rams Horn snails are shaped with a spiral shell similar to a "Ram's Horn."

The fact they are translucent means you don't have enough buffer in your tank to support their shell development. They will soon die off, at least that is my experience. I added sea shells to my tank to increase the amount of available calcium and my snails now get much bigger and robust.

Sorry, I meant flat coil as in... they're shaped like an electrical stove burner (or ram's horn ) - they're flat on both sides, and viewed from the side they're round. It's probaly not the correct way to describe them - I can't think of a better way to describe a pond snail other than football shaped (that's what mine look like, at least).
I found this site, which has some good pictures of the different types