There are a zillion add ons for this thing...
Campaign Cartographer Pro - The base program. Has everything you need to generate a basic wilderness map or simple dungeon. Covers the basics.
City Designer Pro - An add on that focuses on towns, cities, and villages. Includes elements that allow for the creation of orc, elf, and other theamed settlements. Includes tools to random generate the buildings on a street when you want to fill in a big city quickly.
Dungeon Designer Pro - An add on that focuses on subterrainian areas and building interiors. Lots of furniture, traps, statues and such. Includes a rapid room and corridor generator which speeds underground mapping considerably.
Character Artist - Add on. Lets you create simple "paper doll" style character illos. I'd pass on this if you are more interested in mapping.
Dioramas - Add on, but requires lots of specialized functions. Lets you create limited 3-d views of your maps. Very cool, but a whole new learning curve. I'd wait on this one.
Perspectives - Create 3/4 view maps (kind of like those late 2nd edition ones from TSR). Very cool, but like Dioramas there is a whole new learning curve. Walk before you fly.
Symbol Packs (Overland, Fantasy Floorplans, Modern) - Bundles of Hundreds of additional symbols. When you are sick of using the same mountains, trees, and such over and over again, these bundles give you other options. Easy to get confused by all the options when you start, but once you have a half-dozen maps under your belt you *will* want the additional symbol choices.
Cosmographer - Add on, A bunch of extras that assist in creating space stations, starships, sector maps and "Computer Readout" style planetary maps. Very cool, but not what you are probably looking for.
And in it's own category, Fractal Terrains.
This very cool app lets you random generate planet-scale maps complete with basic weather, climate, and rainfall. You can edit the planets, then export them into Campaign Cartographer for more detailed mapping. A standalone app, you can use this for basic landmass generation.
Awesome for sci-fi games where you need lots of planet maps. Also good for fantasy world generation. I'd master CC2 first, but I'm a big fan of this program.