I list some of the management levels and specialties below because they seem more common to certain species of geek, but I see all varieties of geek in the full range of IT jobs. The key consideration in each of these descriptions is the level and variety of geekery each displays at work versus what they do at home.
Jekyll 'n Hide
The most pure is the monomaniacal computer scientist who got interested in processor and systems design, or software development in middle school or high school, focused on CS and mathematics all the way through college and/or grad school on the way to a career in academia or, in the corporate world, as either the Hermit Programmer Guru or team-leading but individual-frustrating Software Architect.
Those who came to the light of IT late in their academic careers but are no less dedicated to technology might begin as sysadmins tending herds of peripatetic servers, growing eventually into specialists in networking, security, storage or other fields, or even to manage entire data centers.
Members of both groups focus almost exclusively on the variety of information technology they specialize in and often don't exhibit the multi-disciplinary geekhood most common in those whose work is less specialized.
They even have hobbies more comparable to civilians than to other IT people and relatively few deeply demanding information-based hobbies like anthropology or quantum physics.
They geek out at work, often even uber-geeking. At home they behave like normal people – trailering the boat to the lake to fish for small-mouth, for example, rather than leaving the boat in the garage because it's partway through an electronics refit that center around systems such as self-designed depth and location sensors that work but appear to violate FAA regulations and some laws of physics.
Compulsive hackery that's completely under control!
That boat project is the kind of project that litters the consciousness, work areas and living space of a more compulsive species of geek, who works in IT because the requirements fit within his or her natural proclivity to fix, disassemble, improve or break every piece of technology they touch, whether it's their responsibility to do so or not.
You can identify those with the most harmless intensity level of this compulsion by the way they interrupt conversations with DMV clerks, airline booking reps, administrative staff at the doctor's office and anyone else they encounter to ask about the laptops, software, scanners, cool mouse, hinky looking printer or other bit of technology that person uses every day.
Left alone in a room, light-duty compulsive hackers will go under tables or desks to find out what hardware is running, or turn on and play with unfamiliar machinery in the room.