I’ve heard of different types of bicycles; Hybrid, mountain etc. I’m super confused, what is the difference between bikes with thin tyres and the ones with thick tyres and rough threading? also whats a hybrid bike/mountian bike and which will be best for someone who lives in a rainy city with alot of tared roads and stuff? (sorry i’m asking so many questions in one 
A good way to explain the differences between types of bikes is to think of them like you would think of cars.
Road racing bikes- These are the bikes you see with the extremely skinny tires and drop handlebars. The people riding them are often wearing tight lycra clothes. These are like the formula one cars or the supercars of the cycling world. They are highly specialized to only do one thing- go fast on paved roads.
Old ten speed bikes- These are what most Americans think of when they think of ‘normal’ bikes. Some have drop bars like racing bikes, some have other styles of bars. The tires generally aren’t wide, but they aren’t super skinny either. They are inexpensive bikes for just normal riding around. They are pretty much all found on the used market these days. These are the 1991 Honda Civics of the cycling world.
Cruiser bikes- These are the bikes with more curvaceous frames, like an old schwinn from the fifties. They are generally, as the name suggests, just for cruising around and having fun. Some love the style, some don’t. These are like the lowriders of the cycling world- not for going fast, just for having fun, and looking good to those that like the style.
Touring bikes- These are very stout road bikes. Many have drop bars, but certainly not all. They are designed specifically for long distance trips and carrying loads in comfort. They use wider tires. You can generally take a touring bike off road on well groomed trails and fire roads. They are like the suburban or the range rover of the cycling world.
Mountain bikes- These are bikes that are designed specifically for off road use. They generally have tires with a knobby, agressive tread. The cranks are higher up off of the ground for more clearance. Many have some sort of sprung suspension, either at the front or at the front and rear. They aren’t as good at riding on the pavement. These are the Jeeps of the cycling world, ranging from a stock jeep to one of those heavily modified lifted jeeps with mud tires.
‘Hybrid’ bikes- These are a cross between a mountain bike and a road bike. The goal was to create a bike that was the best of both worlds, but for the most part, they are the worst of both worlds. They aren’t as good on the road as bikes specifically designed for paved road use, and they aren’t as good off road as a mountain bike. They are like the crossover SUV’s of the cycling world- pretty much not very good at any of the tasks they were designed for. They are often also known as ‘comfort’ bikes, but they are only comfortable for short, occasional rides.
Cargo bikes- These bikes, such as the xtracycle, the surly big dummy, the kona ute, and the madsen kg271 are designed for carrying a LOT of stuff around. They are like the pickup trucks of the cycling world.
Cyclocross bikes- These are very similar to road bikes, but they use tires that are more akin to mountain bike tires. They are used for a form of racing called cyclocross racing. These are like the rally cars (subaru impreza, mitsubishi evo) of the cycling world.
Roadster bikes- Often called Dutch bikes or English bikes, these are large, very heavy bikes designed for transportation. The most notable feature of this style of bike is the fully enclosed chaincase around the chain, so you don’t have to worry about snagging your pants. They generally last forever. They are very comfortable to ride. These are like the old large sedans of the cycling world.
Tandem bikes- These are ‘bicycles built for two,’ although you can also get them for three or four people if you are willing to shell out LOTS of loot. These are kind of like the minivans of the cycling world, in a loose sense.
Folding bikes- These are small bikes that fold up so you can put them in the trunk of a car, or take them on a train or bus, or stash in the corner of your office at work. They are like the smart cars or the geo metros of the cycling world.
I am sure I have left something out here, but those are the basics.
In general, wider tires are more comfortable. Smoother tires give better performance on the road. You can get wide smooth tires, just like you can get skinnier knobby tires. If you live in a place that is rainy, the most important thing you can have on a bike is a good set of fenders that cover as much of the wheels as possible.
Good luck!