I think of it in terms of how a much a person gets into their hobby. Like a computer geek I would think of someone who doesn't work in the computer world, but always has the biggest most recent, knows all the terms, knows the ins and outs can answer every question, just like if it were their profession.
There are baseball geeks, football geeks, computer geeks, model train geeks, cooking geeks.
So if I want an answer to something or want to know about something a "geek" would be the best person to get the answer from. Many times if you go to the individual who has that topic as their profession you get to much of technical answer and they may not know how to translate that into laymans terms. A geek can get you that techie answer, but can also translate it so you can understand it and they'll also usually be much more willing to help.