So you want to pedal to work. What kind of bike do you need?
Any reliable bicycle will do. As long as everything is in good working order and you feel comfortable on the bike, you are fine. If you find an old bike in your garage that may be suitable, take it down to the bike shop for a tune-up ($50 or so), and ask them to evaluate it for regular commuting. They may suggest you replace worn wheels and rusty chains, things of that nature.
If you have to buy a bike, there are three or four kinds of bikes sold to adults in most North American shops: Mountain bikes, touring bikes, hybrids, and what I call "granny bikes."
If you are only using the bike for commuting, forget about a mountain bike. Mountain bikes have fortified frames, and fat, knobby tires designed for rough trails and off-road riding. In fact, on paved roads mountain bikes will be counterproductive, because their weight and the extra friction from the fat tires will slow you down.
Of course, some don't really care about this -- they get mountain bikes to project the image that they dabble in off-road activities, or are a little on the wild side -- kinda like your average city/suburban SUV driver.
Still, they are tough, and in certain commuting situations -- through parks, potholes, or curb-jumping -- they're your best bet.
These are the speediest option, thanks to their light frames and thin tires. Up until the mid-80s they were commonly known as "ten-speeds," but most newer models have 24 or more speeds.
If your commute takes you on long stretches of road without any interruptions, and you can build up a lot of speed, you may want to get a touring bike.
However, if you are driving through city streets and have to deal with red lights and frequent intersections, a touring bike is probably not the right choice.
Touring bikes are also not meant for mild off-road wear (such as detouring on a dirt path) or jumping curbs, which can damage the bike.
I have also found them to be slip-prone when encountering sand on pavement, which is a common situation in the spring in northern climes.
Hybrids are like mountain bikes with narrower tires and lighter frames. I think they're perfect for commuting, and even for semi-long tours (I've managed a 100-plus-mile ride on mine). They're rugged enough for mild off-road detours, and for traversing curbs, potholes and roots, yet can build up a fair rate of speed on the straightaways.
I don't know if this type of bike has an official name, but this is what I call them.
This type of bike is the descendent of the solid model you might have seen an elderly grammar school teacher riding in the 1950s. They have curved handlebars, wide seats, a simple three- to ten-gear transmission, and mud guards. A few even come equipped with lights and baskets. Recent models tend to be black and ruggedized a bit, with wider tires and stronger frames, kind of like a hybrid lite.
They are subtly ubiquitous in university areas, and even overseas. They are also entirely practical and maybe even right for you if your commute is a short one, or you don't need to go that fast.