A Hackerspace is by its very nature very hard to define, its a physical place where individuals and groups can get together and work on projects, learn new hobbies, help out with projects of others, and have fun making things. In the Spirit of Make Magazine, A Hackerspace is more about getting out there and finding people and resources to help you envision and build your projects.
Think of it as a bit of your own big garage full of fun toys. Or like an artists studio with more soldering irons and less pretension!
Want to learn more? Check out http://hackerspaces.org/wiki/