Well, dry and liquid are different types too.
The reason there are so many different types is that yeast are microorganisms that have been cultured by brewers over the course of hundreds of years. In different locations, in the hands of different brewers, in different climates, strains of yeast that behave very differently have evolved. So, the yeast used to make a Belgian abbey ale, while it is an ale yeast, and of the same species as the ale yeast Sierra Nevada uses, will make a beer with an enormously different profile.
Some yeasts are very clean. Some are very estery. Some make banana-flavor in your beer. Some take forever to ferment. Lager yeast is uniquely adapted to cold fermentation and makes a very clean beer.
It's like how there are all these different types of apples. Same species of tree, but a Granny Smith doesn't taste like a Jonathan.
The descriptions at morebeer.com are pretty good. Bottom line is there are a ton of different yeasts that create very different flavor profiles and you use them to make different kinds of beer. You'll only really be able to sample the different types with liquid yeast because manufacturers like White Labs and Wyeast go to great lengths to maintain master strains of different strains cultured from different beers around the world.