When you start your shopping you will find there are generally five bike categories (exempting kids bikes and specialty bikes like tandems).
These bikes are designed for off road and rough terrain biking with
chunky, bumpy tires for good traction
straight across handlebars for better handling
low gearing for hills and dales
front or full suspension to cushion the ride.
Plan on spending from $400 for basic trails to $700 or more if you are interested in serious off road terrain, sharp climbs and just as sharp descents.
Straddle the bike and make sure you can stand comfortably. With mountain bikes you may need to stop quickly and will need all the balance and traction your feet can afford.
The frame is important here, in that you need enough room from the handlebars to the seat for comfortable standing and pedaling.
Suspension comes in two varieties, something like front wheel drive and four wheel drive on your car. Most of these bikes include front suspension to cushion the terrain. Full suspension makes for quick descents but increases cost and weight, and you are going to have a harder time pedaling.
Road or racing bikes
These bikes are designed for aerodynamic riding or racing on the street with
skinny, smooth tires
curved handlebars to increase speed
a light frame
There is a spread in the cost of these bikes, so seriously consider your needs. If this bike is for heavy duty racing or cross country riding, $1000 is not out of the question. Scale it back to $500 for a bike that will last a long time and offer good braking and smoother shifts. If you are an occasional weekend rider, you can find a good bike for around $300.
Comfort is a high priority with these bikes. If you don't like the extreme bent seating contortions of some racing bikes you can get a bike with a higher handlebar. Seats can be exchanged for larger saddles with deeper cushions or anatomic designs. Another consideration is the distance between the seat and the handlebars...too long a distance and you will be straining...too short a distance and you will feel restricted. Make sure you can move your hands on the handlebars and that they are not receiving all your weight.
Unless you plan to compete consistently, there is no need to opt for the uber lightweight bikes. They will shortchange you in stability and endurance. If you are over two hundred pounds, don't even think about it. Also keep in mind the tires on a racing bike can be anorexically thin and demand a higher skill level.
These bikes are designed for mountain biking and pavement riding and are a marriage of the two that might fit your needs. However, the hybrid is not designed to be a perfect mountain bike, nor a perfect racing bike . Picture a mountain bike with
slimmed down tires
higher gear levels
These bikes are designed for the road, the beach boardwalk or very casual riding. Look for a bike that has
chunky, smooth tires
no speeds or three speeds
back pedal brakes or hand brakes
upright seating and handlebars
These bikes are designed for a comfortable ride without neglecting high performance. Look for