There are five basic types of RV. I say basic types, because there are differences within each type that will affect the size, and options that come in that RV. Now that we are clear, let’s start the list.
Class A
The Class A is the largest RV that can be driven. Class A RVs come in two types, gas and diesel, sometimes called “diesel pushers” because the engines are most often in the rear. They can be as short as 27 feet long or as large as 45 feet long. Gas class A’s use a truck chassis while the largest Diesels are built on a bus chassis. Class A RV’s will almost always come with slide outs in the sides of the body that, when extended, increase the living space. These RV’s will have everything that you will need to live for an extended time, basically, all of the comforts of home. These all drive like buses, and due the large size, can only park where buses and trucks park.
Class B
The Class B is a small RV built in a van body. They can have everything you need to live comfortably, but everything will be very compact. These will usually have a murphy bed or, a seat that converts to a bed, a small dorm style refridgerator and a wet bath. (A wet bath has the toilet and sink in the same space as the shower). These have the advantage of being able to park anywhere cars park. The largest class B is called a B+ and has a small box type rear that has more space and can better options and comfort.
Class C
Class C 
Super C
The Class C is a medium sized RV. It has a van front end and a large boxy rear that extends over the cab. These are built on a full size van chassis and driving them feels like driving a very large van. These will have all of the comforts of home but can fell more like a studio apartment. Some will have a seperate bedroom, but many will not. Most will have a bed over the cab as well as the larger rear bed. These are very popular because there are easier to drive than a class A, but larger and more comfortable that a class B. There is also a larger type of class C RV called a super C. It is built on a truck chassis, but unlike the class A, which is on a truck chassis and looks like a bus, the super C looks like a truck in the front.
5th Wheel
The 5th Wheel is a trailer and is usually quite large. These are definitely a home away from home, with everything that you would find in a small house. Many have kitchen islands or small garage areas big enough for motorcycles. These require a tow vehicle, usually a heavy duty pick up with dual rear tires, and special equipment installed in the truck bed to hook up to the 5th wheel for towing. They tend to cost less than the class A, B, or C, but not by much once the tow vehicle is added. The 5th wheel is popular because it has the space and comfort of the largest class As, but at a fraction of the cost. Like the class A, this will also require a large space to park in, but unlike the class A, the tow vehicle can be used to get around.
Travel Trailer
Pop up tent RV 
Travel Trailer
The travel trailer is a small to medium sized trailer. Unlike the 5th Wheel, no special equipment is required for towing, just a standard trailer hitch. The size of the travel trailer will determine the size of the vehicle that is required to pull it. Travel trailers can be as small as a pop up tent camper to the size of a small 5th wheel. The size and price will determine what features are included in the travel trailer. These come in so many shapes and sizes, that it would be impossible to cover them all in one article. Since these tend to be inexpensive, they are a popular starter RV for many. Most travel trailers are small enough to tow around town without a problem and once they are parked, you have your tow vehicle to go sightseeing in.
Summary
There are so many types of RV that there is something for everyone. Just figure out what you think you will need to be comfortable and then think of what you want to spend and that will give you a starting point. Each class has advantages and disadvanteges, you will have to make a list of what you think those are and that should make your list even smaller. You will have to look at them in person to really get a feel for what it’s like to be inside one and to get the feel of how big or small it is.