THE BEACH CRUISER
The cruiser will not get you anywhere fast, but it will get you there in style. Case in point, check out this beauty from the Target Liberty of London collection. Keep in mind that the beach cruiser often only has one speed (though some have three), and your body is not positioned for optimum pedaling, so you won't be getting there in a hurry. This bike is perfect if you want to get out there and enjoy the weather and the scenery, and to...well...just cruise!
THE MOUNTAIN BIKE
No matter how many different bikes I own and ride, the mountain bike will always be closest to my heat. It is my first love. If you are interested in trail riding, invest in a good mountain bike. Bikes take quite a beating on the trails, so in this category, more than any other, you get what you paid for. Think of it this way, if your bike can't take the beating, your body will! In the 10+ years Ive been mountain biking, I've seen countless couples where the dude is on a $3K bike and having the time of his life cruising through the trails, while the girl is on a $100 POS that weighs 50lbs, doesn't fit her properly, and handles like a tank Surprise - she looks totally miserable. Check out Giant Bicycles - a mass manufacturer that makes bikes for many of the popular bike brands, so it can offer quality at a lower price. Choose a better quality hard tail (suspension only in the front) over a bottom of the line full suspension. Make sure you test ride, and get your bike shop to swap out the saddle (to fit your sit bones), handlebars (risers will bring the handlebars up for comfort), pedals (if you want to go clipless), and tires (appropriate for your local terrain), as needed.
THE ROAD BIKE
If you are looking to biking as a way of getting in shape, then look no further than a road bike. The advantages of road biking is that you can start right out of your font door, and get a smooth fast ride that gets your ticker going. Make sure you get a proper fit form someone who knows what they're doing. You will be sitting on that bike for long stretches at a time, so if it doesn't fit well you will be in pain. Over time, an ill fitting road bike can lead to unnecsary wear and tear on your body. Again, Giant Bicycles will offer you good value for good quality.
THE COMMUTER/TOURING/HYBRID BIKE
Most people fit into this category when looking for a bike. This is the all purpose bike with a geometry somewhere between the road and mountain bike. It will take you to work, the grocery store, on a breeze through the park, on the road for a little excercise, or on a bike tour of Italy. The custom built Rivendell Atlantis is king of this category. However, if you will be parking your bike on the street, then consider getting a cheap and UGLY commuter - it is not likely to be stolen. In this case, a used bike often offers decent quality with a reduced price tag. I know many people who have an ugly/cheap commuter for work/school, and a swanky bike for everything else.
THE SINGLE SPEED
Just like the name implies, this bike has only one gear (you get to choose the gear) - no changing to an easier gear when you encounter a hill. A single speed is not just a kind of bike, it is a life style. In mountain biking, the single speeders are the most hard core and excellent technical riders. Out on the street, a single speed bike is hip and trendy. It's great for an extra challenge on your morning commute, and maintenance is a breeze. For an everyday single speed, I love the Bianchi San Jose. I quite enjoyed cruising one of these everytime I house-sat for a mountain biking friend, with his blessing, of course.
A BIKE FOR YOUR TYKE
If you are looking to buy some lucky tyke their first wheels, then look no further than a balance bike. This one by Skuut looks funny, has no pedals, and is wooden, but it will teach a child how to ride a bike without the need for training wheels. This bike teaches the child how to balance on two wheels. They start out walking on either side of the bike, but as they get more confident, they will start pushing off with both feet and gliding - this is a natural progression because even little tykes want to go fast! Once they perfect the balance bike, a kid has no problem with a regular two wheeler, because pedaling is the easy part.
And no matter what bike you choose - PROTECT YOUR NOGGIN AND WEAR A HELMET!
A COUPLE OF NOTES:
WOMEN SPECIFIC BIKES
Most manufacturers now make women specifc bikes. These optimize the bike's geometry for women, who tend to have longer legs and shorter torsos than men. Women specific bikes are the way to go if you are petite. If you are taller, go by fit. I am 5'10'' and I ride a men's frame. Women's bikes do often come in more pastel colors, which is controvesial. Some girls say that they hate pinks and pastels, and want 'normal' colored bikes just like the boys have. I think pink bikes are awesome - especially if the lady on it is a badass!
CHOOSING A BIKE SHOP
I recommend that you research your bike options online, and then buy your bike at a local bike store. Find a shop that has a good reputation, and carries the type of bikes and the brands you're intersted in. You may have to shop around to test drive different brands. Keep in mind that the shop can swap out parts on the bike before you buy it so that it fits you better or suits your local terrain. Though this may carry an extra charge, it will be cheaper than replacing those parts yourself. When you buy your bike at a reputable shop, they will remember you and build a relationship with you. This means that they may offer you discounts, and they may even do simple repairs/installs for free. So choosing a bike shop is much like dating - can you see yourself in a long term relationship? I choose on the basis of the brands they offer, their offerings in the chick section, and the vibe - no snooty bike shop need apply.