1. Windows refugees. These are the folks who've quit Windows for one reason or another, and are never looking back. Maybe they're fed up with Product Activation, the closed-endedness of Windows, its draconian licensing (compared with Linux, that is), or any number of other things. The exact level of technical sophistication for expatriate Windows users who switch to Linux is quite diverse, from what I can tell. I get reports of people in this rubric branding themselves as everything from Joe Sixpacks to experienced hackers.
What I generally have a hard time figuring out is how many people who leave Windows for Linux stay there. That's a statistic that's probably about as hard to cough up as how many Linux users there are, because it's virtually impossible to get reliable numbers. (What is clear is that the people who leave Windows and don't look back will not hesitate to speak up.)
2. Longtime devotees. These are the folks who were with Linux from the git-go -- or at least for long enough that it scarcely matters. Many of them left Windows before it became the monster it is today, or were Unix (or BSD) gurus from before. *NIX metaphors are what they're comfortable with, and they typically disdain Windows on general principles.
3. Absolute beginners. Each year there's that many more people who don't know jack about a PC and get their first taste of it through some flavor of Linux rather than Windows. They may pick up Windows skills along the way, or have both machines around when they do learn, but they at least get some exposure to it right from the start.
4. Cross-trainers. People who use another OS as their primary OS, but who consciously build as much of an acquaintance as they can with Linux, could be called cross-trainers. They probably haven't committed to doing all their day-to-day work in a Linux environment, but they're curious about it and want to get as much exposure to it as they can.
Obviously these categories are not wholly closed-ended. Someone can be both a Windows refugee and a cross-trainer -- maybe they left Windows for personal use at first, but now use both it and Linux for separate tasks. Think of them more as starting points rather than end points.
Of all four categories, I'm probably in the last one -- I have a notebook that I dual-boot or re-image with Linux (depending on the circumstances), and a bunch of Linux virtual machines. (I don't depend on VMs to give me any ideas about performance or real-world behavior; they're just for convenience.) For day-to-day stuff I still stick with my Vista box, though, but I'm learning as much as I can about workaday Linux behavior by doing similar tasks in parallel -- dealing with e-mail, for instance, or working with documents.
Do you see yourself as fitting into one of these categories? Or are there others I might have overlooked? Let me know what you think.