First of all, we have to look at the four main categories of bicycles. Each category has its own characteristics and advantages. What will determine our final choice should be the use and purpose that we want to buy a bicycle.

Road Bike (road bike).
The racing bike. Previously most of us used to feel fear and awe with the idea to use a racing bike. Partly justified because everyone saw racing bikes only on TV fighting in one of the renowned cycling races.
They are made to run on asphalt roads and to do it quickly! They have very thin tires, lightweight frame and the rider should get a stooped posture on such a bike. At first it may be inconvenient but very quickly everyone can get use of it.
If our main goal is the long distances, rapid ascents and generally the speed, then this is the bike you should buy. With such a bike we can really feel like cycling exercise, and certainly with such a cycle we need to participate in road cycling races. The cost can range from € 400 up to € 18000! One key factor that determines the cost is the weight of the bike. The more light the more expensive. However, the lighter is the faster!
Most economic models are made either of steel or aluminum. The more expensive models are made of carbon fiber (carbon) or titanium. A general advice is to purchase as much as possible light bicycle as you can afford.

Mountain Bike (Mountain Bike).
The most popular bike. With thick and gears and a different frame both in shape and in size. For these reasons it is much slower than the road bike. But it has the advantage of much more comfortable position of the rider due to the geometry and the straight handlebar.
Ideal for all surfaces, necessary if we are interested in cycling on gravel and mountain paths. With such a bike we can participate in mountain bike racing. There are several subcategories that have mainly to do with the suspension and weight. A mountain bike can only have front suspension and/or back too. A good bike has brakes for lightweight or more powerful disc brakes front and rear. There are cross country mountain bikes mainly for racing cross country, the trail bikes which are the most prevalent after doing almost everything. For more advanced people who want more excitement as fast descents full of adrenaline, there are the Enduro and Downhill where everything is bigger and therefore more boring but much more resistant ready even for jumps in motocross style!
The cost is again a function of the quality of the construction material and the quality of components. Here the weight is not the only criterion since in certain cases the burden is a prerequisite for a strong bike that will withstand rugged trails. General advice for this bike category is important to the quality of the main parts (suspensions, brakes, gears).

Hybrid bike.
The most popular option in the last 2-3 years. It is something between road bike and mountain bike. Faster than a mountain, most boring and slow from a road.
Wearing thinner and smoother wheels of a mountain, with air pressure almost as much as in a road, so it is quite fast.
The handlebars are straight and the position of the rider more upright and comfortable. Most are made of aluminum and have brakes V-brakes (tires).
They have 16 up to 27 speed options with 2 or 3 main gears 8-9 forward and back.
If you buy a bike for shorter journeys and for travel to the city or local cycle paths a hybrid bike is ideal as it offers us comfort, speed and flexibility.
Our advice in this case is to look for a model that is available or better integrated various useful accessories that will help us when we go school, university or work, or even shopping. Such accessories are the grill back, the running lights, the locks.

Cruiser Bike (City Bike).
You will find such a bicycle to circulate as everyday transport in large and advanced cycling countries.
It is very popular among women since it is extremely comfortable and easy to use. It has large wheels with a relatively wide rubber band, wide and comfortable seat, and a raised wheel with grips parallel to the frame, which makes the driving of such a bicycle extremely pleasant. Most are made of steel or aluminum, and usually have only one gear.
If the transport in the city in short trips is the only reason you want to buy a bike, then it is the ideal.

Available accessories start from the basics (grill, headlights) and are moving up in shopping baskets even special seats for children!
The advice in this case is to be absolutely sure that we do not want to do nothing more than short trips in town with our bike before entice us admittedly nice appearance of a cruise bike.
There are of course other types of more sophisticated bicycles like folding bike or the velocipede. What should beware before our final decision is to be sure for the use for which we want to buy a bike, try to climb on the bike of interest (we can borrow from someone who already has) made a sizable route to visit specialized shops bicycles, and finally to avoid the temptation to buy a cheap bike. The last thing we would want would be to stay in the middle of the road due to a failure cheap component. Happy trails!