There's no rules or limitations, but always include an action and sometimes a result. The result may be points (+100 points, -infinity points) another action (lose turn, shave your mustache) or anything else you can think of. I've posted a card as an example.
You can use your card to reply to someone else's card (to “beat” it) or you can make a standalone card.
You're allowed to use color or just black and white. You may use photoshop or mspaint or even scan something in. You can do serious drawings or just stick figures. It's all up to you :)
For a more complete description, check out the wiki link here.
