From the game guide

Game Stages

While playing Spore, you will go through five different stages of gameplay. Each one of these stages will have a very unique gameplay — some stages resembling pac-man, while others focus on exploring the universe.

1. Tide pool phase
2. Creature phase
3. Tribal phase
4. Civilization phase
5. Space phase

As the game begins, a meteor is shown headed towards an unexplored planet, subsequently crashing on its surface. This meteor contains very simple cellular organisms - namely, you and your competition

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  Tide pool phase

  In this stage of the game, you will start off as a microscopic organism fighting for survival. You will direct your creature to move around a 2-D landscape in order to eat things that will make you grow, and run away from things that will hurt you. Luckily for us, Jenova Chin has created an almost exact replica of the game play in Flow in Games. It’s a great demonstration and shows the basics behind this stage at which you will continue to grow until you access the creature editor and can make your creature evolve.

  
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  Creature phase

  At this point your main goal is to rapidly evolve your creature to his final stage in a 3-D world. You will continue through the game, eating enough food to access the creature editor where you can add body parts and shape your creature. At this point you can choose to move your creature out of the water, or remain to build an aquatic species. You can choose to give your creature a spine or not, it’s all up to you.

  Most of your time during this stage will once again be avoiding enemies and finding food. You can experiment with different types of limbs and body shapes in order to make the creature you want. You can choose to make an aggressive creature that fights his way through the world, or a passive herbivore that runs really fast.

  
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  Tribal phase

  You enter the tribal stage once the genetic structure for your creature is complete. At this point, a few of your creatures gather and you are no longer directly in control of your creature. Instead, you control groups at a time assigning different jobs for your individual tribe members. Additionally, you’ll controll the environment around your creatures – allowing you to give your tribe gifts, like spears or baskets.

  The goal in this stage of the game is to continue building up your tribe of creatures until their hut (the main living quarters) advances far enough that they can start building other buildings and move into the civilization stage.

  
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  Civilization phase

  Now you enter into the point in the game in which your city starts interacting with other cities and creatures. You can maintain peaceful relations, or enter into wars with other colonies. Your job is to keep your city the dominant one and conquer the world, one city at a time.

  
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  Space phase

  Once your civilization has advanced far enough, you will be able to purchase a UFO. This UFO will be able to leave your planet and scout other planets. You can use it to populate other worlds with your species, or build a completely new ecosystem somewhere else. The UFO has tools that allow you to terraform other planets so that they’re suitable for life. You can even abduct different creatures from your world and plant them on new ones to see how they will react.

  After you experiment around with your local planets, you’ll be free to go anywhere in the universe. You can take your UFO and travel to new galaxies and make contact with alien species. Whether you decide to make peace with them, or destroy their world is up to you. However, beware that there will be consequences for destroying other civilizations as they will have intergalactic allies that may retaliate against your civilization.