Basically, you have to earn a certain number of Partner Points to achieve the different levels of Partnership, which you earn through having people get certified, customer feedback, getting products certified, etc.
Then, depending on your level and whether you earn any designated competencies, you get an MSDN Subscription, Technet Subscription, and lots of software licenses for internal use (such as O/S, Office, Visual Studio), which makes partnership a great deal financially.
You can also use the Partner Logo on your Web Site, advertising, etc. I think this gives you some added credibility with potential customers. You also get access to technical resources and information, which is valuable.