The object of admiration at your workplace. While he or she may be someone that matches your taste, is more often than not a person that you would have never given the time of day in any other situation because of their looks, their personality, their work ethic, etc. But with a regular 35 hour workweek (or more, say if you're a lawyer or investment banker), you're seeing some of these people more often than you sometimes see your friends or significant other, and, well, humans are wired to form attachments. Also, most office environments have some form of dress code, so your workmates tend to be somewhat groomed and presentable, which makes them more attractive than 80% of the people out there that you never see.
At its most enjoyable and risk-free, a work crush is someone that, at the very least, makes you look forward to going to work. They makes work a bit more bearable. Sometimes they're someone you can talk (or flirt) with, sometimes they're just someone you like looking at; optimally they're both. (In some cases, not talking to them is the optimal situation, especially if they are high-maintenance/flighty/mean-spirited/ditzy.) If you do interact with them, then this generally happens if you or he/she are just generally friendly, naturally flirty, or there is a clear understanding that it's all a game, nothing more. It's all in good nature and nothing is meant by it. You work, and the moment you clock out is the moment you forget about them.
At its worst, it can be an all-consuming, ugly obssession and leaves you feeling gawky, awkward, and stupid. Usually reserved for cases where the person you have a crush on is completely not your type; this is probably because the physical/mental attraction hurdle that you have in the first place is wide enough so that the very act of successfully leaping over it means you have pretty serious feelings.
The real problem begins is when both and your crush realize that the attraction is mutual. Conversations become awkward and filled with innuendo, real or imagined. This is when your inner Drama Queen comes out and starts overanalyzing everything that anyone says. God forbid that both of you should be stuck in the office... together... late at night! More drama occurs if:
Either of you are taken (or both are taken but it's usually more drama when only one is);
One of you is clearly higher in the status ladder than the other (e.g., boss/secretary; partner/paralegal, associate/cleaning lady);
Situation 2 with the addition of a serious corporate ladder climbing workplace;
Interoffice relationships are strictly forbidden by your company;
Either you or him/her are the gossipy type;
All of the above.
General rule of thumb is that getting into an office relationship is a very bad idea. Should the relationship end in a sour note, you won't be able to march out the door, warm in the knowledge that you'll never see them again. You will see them again (the very next day, even) and Murphy's Law will come into full force to make sure that you see each other often at work even if it was originally the case that sightings at work only happened once in a blue moon. This rule is more applicable in situations where you are working at a company with the intent to stay long-term; less so if you know that you'll be out the door within a few years or so. There are exceptions; I've heard of plenty of marriages and happy relationships starting this way.
If you are planning on breaking this unsaid rule, best piece of advice I can give is to be discreet. If your relationship is happy, it's good, but there will be bad times and bringing work into your personal life and your personal life into work is just never a good idea. Even if office relationships aren't forbidden, even if you break up cleanly and without sour feelings, everyone else will be walking on eggshells. There is nothing, nothing more loved at work than office gossip. People who you've never met in your entire life will be dissecting your relationship from all angles. And people will be talking about it years and years later. True story: a few months ago I sat in with some secretaries for a birthday lunch and was treated to stories about a certain secretary who drunkenly forced her attentions on some married attorney in the men's bathroom at a Christmas party from five years ago. Now, this secretary is a nice girl and all and I can't quite imagine her doing something as silly as that (even with the assistance of heavily spiked eggnog) but the fact that they were still talking about it five years later is something that you really need to keep in mind. Nothing like being known as "the one who made out with So-and-so in the kitchen when they should have been saying farewell to So-and-so instead" for the rest of your work career.
P.S. If things go very badly for you in your relationship and you are a natural drama queen, you can fictionalize your life and make it a successful best-selling book, I suppose. Wasn't half of Bridget Jones's Diary about Bridget maundering about her work crush?