The berth of the caravan is the total amount of people that the caravan can sleep. Now obviously that is open to interpretation as you could probably squeeze considerably more people than just the two in the front make up bed, but for this purpose we will stick with what the manufactures state the beds are designed to do.
But a big mistake people make is the assumption that because they may want to only sleep two, or four people in the caravan that they need to go for that particular berth of caravan.
Here are the different berths of caravan and what they are designed to do.
These caravans are designed to be smaller and therefore lighter in weight and some would say easier to tow, (but I wouldn't alow this to influence a purchasing decision) and come in only a single axle format. They cater for a 2 people with a bed that can be made up from the front seating area, normally into a double, but in most cases also two singles. The reason for this is although the caravan is smaller than most other beths of caravan, due to the layout there is often a greater feeling of space and a longer front parallel seating arrangement gives the advantage of being able to be used as singles.
There are two main 2 berth layout types, the vast majority will have a good sized washroom at the rear, center kitchen and front parallel seats.
But some will be smaller to cater for an even lighter tow weight by having an end kitchen, a smaller side washroom, but still have a good sized front living area,
These are by far the most popular layout types as that cater for the majority of customers needs. (Although they can cater for four people, if you actually need to sleep four people for the majority of time then I would look at six berths) There are two distinct categories of four berth caravans, the fixed bed and the non fixed bed, both of which are available in the larger twin axle format, for that advantages and disadvantages of twin axle, see here.
The four berth fix bed layouts, as the name suggests, have some form of fixed bed, a washroom, a kitchen in the center and either a front parallel seating area, or in a few cases a front 'L' shaped lounge. for a more detailed look at fixed beds click here.
The second form of four berth does not have a fixed bed and is an extension of the two berth's featured above. They have a large end washroom, side dinette which is used as a seating area during the day and can be made into twin bunk beds at night, a center kitchen and a large spacious front dining area which doubles up as a double bed or twin singles at night. These layouts are for those who think a fixed bed is a waste of space, they are available in a twin axle, but are are fairy rare.
I wont go into five berths as its a bit of an old layout and we don't see them so often now, they did consist of a double dinette layout with a central washroom opposite the kitchen area, they were fazed out in the late 2000's
These again come in two distinct layout types, the first one is the original which incorporate fixed bunk beds obviously aimed at the family market. They normally have two fixed bunk beds with a further two that can be made up out of a similar side dinette arrangement as the four berths above. In recent years that has been a move towards a triple bunk arrangement but this has lost favour as the third bunk was normally on the floor which most people weren't keen on. Some manufactures made these in a twin axle, but we don't see them that often. This layout below is the newer style take on the six berth fixed bunk layout and gives a separated area for the kids which can be shut off at night. 
The second six berth layout only comes in a twin axle format as it needs the extra length a twin axle gives the caravan. It has a fixed bed in the rear corner with a washroom next to it. There is a side dinette which makes into twin bunks with a kitchen area opposite and a long parallel seating area at the front. This is perhaps one of the most open plan and spacious feeling layouts available and is widely used for seasonal caravnning where size and towing isn't an issue.
