Single-Player: Players are in charge of their own lab, where they can learn the game & experiment and create lifeforms to be used in Multiplayer. For Facebook users there is also a social mode, where they can share their creatures, send/receive gifts and visit their friends' labs.

Multi-Player: Real time strategy with up to 4 players, 1v1, FFA and 2v2 modes. You can create your own 'race' by designing your units in Single player, or even live while playing against a human opponent. The goal is to take control of the resources(Food) on the map and come up with better designs of mebas that can beat your opponent's army.
Again you are completely free to experiment, what designs work best, but if you are a beginner its a good idea to put some Antibody and Detector if you want your mebas to fight!
I also made a step-by-step tutorial for the Multiplayer(also available in-game):
http://www.youtube.com/watch?v=ah2RsbpTu3o

Dont forget to share your results with your friends!
Facebook app page: https://apps.facebook.com/themebas/
Game site: http://www.themebas.com/

if you found any bugs, please report it via the official forums at: http://www.themebas.com/forum.html

and finally, dont forget to vote it up:)

Martin

PS: if you are bothered by the menus hiding your mebas, you can close all interfaces by the controls on the bottom right (or press ' M ' )
PS2: do NOT set a password if you create a game and want others to find it!