There are several levels of membership. Each higher level includes the lower level's benefits, and gives you a few more.
Free membership includes access to the forum, access to most of "Quick Action" mode, and access to in-game observer cameras.
Basic membership costs a monthly fee. It includes access to the "Take a Host" mode.
Intermediate membership costs a little bit more. With this level of membership, you get access to any creature that already exists on the planet. It includes a comprehensive listing of all creature types and concentrations. If you have enough points, you can use them to get access to any single NPC creature in the game.
Lord membership costs a little bit more. If you have programming skills and you want to add new features to the game, get this one. It includes access to the game's custom development suite, and the ability to ask other developers for help. You also get to rent a small area of land to do with as you wish. With this land, you may edit the physical laws that apply, such as creature strength, birth, death, magic, realm-limited items, and restricted outside items.
Master membership costs a bit more. You are given dominion over a huge area of space, with power over it all. Don't like that planet there? Destroy it. Don't like that species? Eliminate it. Want a river flowing through your property? It's there. This is a bit tricky though, since you must create a legal agreement with Lord membership players in your region, and not violate it. You also get the ability to teleport to anywhere in your area of space.
Realm membership is the most expensive. You get access to your own dimension, and you populate it with planets and whatnot. You also get access to dimensional portals, which you can make agreements with owners of other dimensions in order to travel there instantly. You also get the ability to teleport to anywhere in your dimension.