Rear Facing Only: These car seats are made for rear facing only, seats are optimized for the first 0-5 years (or shorter in many countries). Using an infant seat from the first day is usually a better solution since kids are so small. Most parents then switch to a rear facing only seat at around 4-9 months.
Even if we don’t like it we have to forward face our kids at some point. This age varies but is around 3-5 years in Sweden, 9-12 months in Germany, and 12-15 months in US. As you can hear, habits can be very different. Typical Swedish rear facing only seats would be Britax Hi-Way, Maxi-Cosi Mobi, or BeSafe IziKid.
These seats typically allow rear facing to around 4 years of age. When time comes for forward facing many select a belt positioning booster, also called a high back booster, or a seat which allow harnessing of older kids. Both methods are approved and offer similar safety. Far from as great as rear facing but still good protection.
Combination seats: These seats may be used rear facing at first and then forward facing with either a harness or seat belt (as a high back booster or belt positioning booster). A combination seat is just what the name says, it’s a combination of two different seats.
The thought is to have a seat which can be used for a very long time. Some don’t like these seats since they are a "compromise". It’s difficult to make a car seat which fits perfect for both an 8 month old and an 8 year old. The combination seats are usually a little larger which has the added benefit of allowing rear facing for a very long time. Disadvantages are usually the higher weight and also guidance of seat belt.
Typical Swedish seats in this category are Britax Multi Tech, Britax Two-Way and Brio Zento. Both Multi Tech and Two-Way are popular, most kids can for example rear face until age 5 in Multi Tech. When the rear facing stage is over, preferably around age 4 , the seat is turned forward and one used the harness or seat belt.