John Tropea has been thinking about teams and communities for a while, and he has a nice article that ponders the differences and similarities. Team-based CoPs compared to cross-functional CoPs
Communities of Practice typically are a group of people coming together to share and learn about a common interest; as well as building a voluntary output of materials. These are usually not driven by management, instead participation is voluntary, and traditionally the goal is about learning and building capabilities rather than performing tasks.
You might guess from the tone of that paragraph that teams are geared toward getting something accomplished. Project teams have a responsibility for successfully getting a project to completion. They have a specific mandate to move work forward for the business. Of course, these teams are made of people who want to get better at what they do. This is where the community comes into play: they are interested in improving the work that the team can do. (Very broadly.)
John's article has much more on this comparison. His original motivation came from teams wanting to employ CoP-like tools and some discussion of whether that makes sense, given what these groups of people need to do.