Spore is basically a combination of several different games that already exist, put in a new package, and sent right back to you. It consists of five different stages where you must complete different tasks to progress onto the next level. You can move as fast or slow as you want to, but the game takes about 10 hours to play all the way through. It has nice replay ability, in that you can basically start all over and make a whole new species. The game also has an interesting online feature where you can share your creatures with your friends and even compete against other players creatures.
The first stage is like flow or fish tank on Addicting Games. The first thing you do is choose whether you want your creature to be a carnivore or herbivore. After that you start out as a cell in a tide pool, and you must either eat other animals or little plants to get points which you can then spend on special features of your creature. When you get tired of that you get your little guy, lets call him Joe, some legs. He crawls up on land, and our next stage begins.
Now you have a Joe that you control in a 3D setting. If you chose carnivore, then your task is to attack and defeat X number of other animals to take their nests. If you chose herbivore, then your task is to make friends with X number of creatures to let them share their nests with you. You can use your acquired nests to produce your next generations with whatever changes you've added. After you have enough nests you move onto the next stage: Tribe!
A group of Joes have banded together and formed a small village. This stage is somewhat like The Sims. You will progress through this stage until you are technologically advanced to form a civilization. This coincidentally, also happens to be the next stage.
This section is like the game Civilization. Your goal: Gain global domination by one of three ways. Economic, where you take over the world through your wealth. Religious, in which you take over other countries of Joes by spreading propaganda.
And the final route, Military, in which you attack other civilizations and destroy them. When you are done with this stage, you will move onto the final stage: Galactic Conquest.
This final stage you will invade other alien planets until you have taken over the galaxy. You can establish colonies on other planets, and eventually terraform them to your needs. For example, you can use a heat ray to warm an ice world up. But if you leave it on too long, it can turn into a molten, charred, and useless planet. After you have the galaxy under your thumb you can move on to other players and test your species against theirs.