At its core, Empire Earth is an old-school strategy game: you gather resources with your citizens, build buildings, muster an army and try to annihilate the opposing force. It has nothing fancy, like experience-gathering heroes (there are heroes, mind you, but they don't advance levels) or (al)mighty artifacts, only sound strategic gameplay.
Yet, it is different from most other base-building RTS games; it neither has a fantasy setting nor is set in some sci-fi world (mostly). It plays out in the various epochs of human history, and on an epic scale, too: the game's timeframe spans from the prehistoric age, when all you can fight with are clubs and stones, to the distant future, complete with mechs and (in the expansion) spaceships. There are 14 epochs in all in the original game, and 15 in the expansion. You may start in any of them, and can set the end to be any of the epochs which come after the starting one (yes, if you're patient /and have a good defense/, you CAN go through all of them in one game ;-)), advancing through them by investing a considerable amount of food, iron and gold in the process.
Like in every old-school RTS, the base of your success is a good management of resources. There are 5 of them in the game (food, wood, stone, gold, iron) and you'll need them all in order to succeed: for example, you use wood for most of your buildings, while iron and gold, along with food, go into training various units. One thing to note is that in this game it's virtually impossible to run out of natural resources: every deposit has 300,000 resources in them, which is a HUGE number. Trees, animals and pathces of vegetables are the only exceptions, but there are plenty of trees and you'll want to change from animals and vegetables to farming as soon as possible anyway.
Speaking of units, Empire Earth employs a rather solid rock-paper-scissors system, with every type of unit being more vulnerable to something, while having no difficulty in eliminating a certain other target. Unit line-ups are mostly what you may expect from an RTS; there are archers, pikemen, various cavalry, catapults and such in earlier epochs, and riflemen, tanks, airplanes and cannons in later ones.
It's not your bog-standard unit composition, though: there are priests and prophets, which give the game a unique flavour, especially the latter. While priests can convert enemy units (and, in later epochs, even buildings!), prophets can call down calamities on unsuspecting foes, such as plague to kill an army in a slow way, or firestorms to wreak havoc among enemy buildings.
There are a number of ways to play Empire Earth: if you want to go single-player, you can play one of the campaigns (by the way, they can take an extreme amount of time to finish), play a scenario or go against AI-controlled enemies in a skirmish, either in free for all or teams. And if you'd like to play multiplayer, you can do that too, over a LAN or over the Internet.
Finally, the AI of computer-controlled foes deserves a mention; unlike those of many other strategy games, it IS a challenge even for more experienced players. Too bad the same can't be told about the pathfinding AI: for example, units are prone to stumbling over each other in narrower places. Still, it does a relatively good job, and it's not that difficult to get used to its quirks.
To speak of the downsides of the game, I think the weakest point of it is the graphics; it's not simply dated, it was ugly even when Empire Earth came out. While it's decent when viewing from a distance, textures (especially those of people) are outright ridiculous when you zoom in on them. So, you'd better watch events from a nice distance, even when not in battle.
Also, the (intentionally slow) gameplay may not be that attractive to people used to today's spiced up strategy games.
All in all, Empire Earth is a great game in that it delivers what one may expect from it. If you like classic real-time strategies, I advise you to check this one out.