Similar with Age of Empires II, but better made!
The improvements come in the gameplay. Many new races to choose from, and now you can make your own custom race. Basically you have 100 points to spend on different bonuses to land/sea/air units or your civilization, etc... A very nice addition if I do say so myself.
There are many more units than Age of Empires II, since Empire Earth has 10 Epochs (Ages), with many new units and upgrades to previous ones. The units themselves can have their attributes upgraded. You can spend resources to upgrade a unit's (all of the same type of unit are affected, not just THAT unit) hitpoints, damage, speed, armor, damage radius and the like.
A few new additions come in the form of Aircraft Carriers which can send air attacks from great distances, but is just about defenseless when fighting another ship.
There are also air bases which can send out aircraft a certain distance, based on how much fuel that unit has. Since planes can nuke buildings, they are extremely useful and downright fun.
The game plays very similar to Age of Empires II, except that it takes alot longer to win a game. If you have even the slightest bit of interest in strategy games I would recommend you get Empire Earth (preferably the Gold Edition which comes with the expansion, or whatever it's called).