By default, here is an idea of the things tracked by Google Analytics (the free one). This is a very non-exhaustive list and only scrape the surface:
- Operating System (and version)
- IP Address
- Location (Easily down to the City, but really it could probably get down to a house if necessary).
- Language
- Browser type
- Screen Resolution
- ISP
There are various other things, depending on plugins, cookies, and so-forth. But that gets you a basic start.