well, i'm glad you asked! you see, mostly i'm working on a Warcraft 3: The Frozen Throne map. and i needed an excuse to plug THE BEST GAME EVER.
anything with "USA" in large letters in the middle.
oh, wait, that was terrible. anyhow, to play warcraft you need the game and also a map. it loads the map, and then what happens depends what the map says. of course the game comes with a bunch of maps. but you can make or download more too.
the main elements of a map are: terrain (like basic landforms). doodads (like decorations). units. a map script (think programming telling it what to do).
also war3 is fully 3D and you can put cameras anywhere, and play pretty movies. and you can put sounds in a map.
ok but you still don't really know what a game is like. cause my description of a map is too general. but, that's part of the magic. maps are such a general construct that you can make drastically different types! some of the main types of maps are:
Standard maps -- these maps have no special programming and let you play the game the official way. you start with a town hall and 5 workers, and harvest gold and wood and use it to build more buildings and purchase units (from your buildings) and fight your opponent. this is similar to warcraft 2, starcraft, command and conquer, age of empires, etc (war2 and starcraft actually had custom map editors too. no idea if CandC or AoE did)
gonna take timeout to mention one cool war3 feature: hero units. they get xp, can carry 6 items, and get 4 spells which they level up as they gain levels. they have 3 ability stats.
RPG maps -- the basic idea is you get to control a custom-modified hero, and so do a few or your friends, and you go around killing pre-placed monsters. possible there is a plot with cut-scenes. there's probably a town (or a few) where you can get supplies and heal.
Open RPGs -- either some game masters use special commands to drop monsters then other ppl fight them, or everyone can use special commands and you're supposed to set stuff up then make your own heroes to fight it. i think they're kinda a bad idea, and it'd be better to just make a map with stuff on it. b/c to get a good map you need to balance test, not fight what took you 10 minutes to place.
cinema maps -- just sit back and watch a movie. :-)
tower defenses -- waves of monsters try to get past you. build towers (which can attack but not move) to stop them. sounds simple but can be quite complex.
aos maps -- the name is just the abbreviation of the name of the poster map of this type (Aeon of Strife). anyway, there are 2 computer-controlled towns that constant spawn waves of monsters and send them against each other. both sides are equally powerful and cancel out. players each get a hero and try to turn the tide of battle. there are also 3-way versions.
vampire hunters -- really there's only one map of this type, but it's awesome. the vampire team wins be destroying the main human city in the middle of the map within 7 days. the human team (hunters) wins by defending that long or by killing the vampires often enough. every player has a hero. to aid them, vampires can collect souls from villagers around the map. hunters try to chase them off and save the villages. vampires have a free teleporter system to get to any corner of the map from any other corner. hunters can only teleport from anywhere to the middle, and that costs them money.
single player campaign -- the game comes with some single player maps that are kinda a mix btwn standard and rpg elements. they're well done.
hero arenas -- fight it out! i think they're dumb.
micro wars -- do you have the skill to beat someone fighting a small num of units vs a small num of units? even if they're the exact same units? it's amazing to see even with the exact same units, the guy with better strategy can win often with half his guys alive.
anyway, my map is an RPG, with custom scripted spells (when certain spells are cast, i programmed it to add certain extra effects), and cinemas, and pretty terrain, and a kinda lame plot, but lots of subquests anyway.
oh and i forgot the best part! battle.net! blizzard's *free* online game servers. all you need is an internet connection and you have access to many thousands of war3 players to play with. if you make a crappy map and host a game, you'll probably get joiners immediately. i certain have no trouble finding ppl to beta test my map. the wait time for ladder (standard games for a ranking system based on how much you win/lose) is like 30 seconds.
also good about battle.net is you could play with *me*! think about it :-)