There are up to three possible seat options for a pram/pushchair. You can use one or two of these or all of them. You don't have to buy all of them.
Usually any pram/pushchair will come with a main seat unit which is designed mainly to be used when it's in pushchair mode - this can be sitting up like a chair or reclined for sleeping and sometimes they go into a fully flat recline. Most pushchairs (if you think of something like a Maclaren or Graco for example) have this built in to the frame and it can't be removed, but with some, usually the more expensive models, you can take the seat unit out and turn it around so that it can be parent facing or forward facing. The main seat unit is usually used until your child is around 3 years old and stops needing a pushchair. If it lies flat, you can in theory use it from birth, but you might not want to if it faces away from you. Otherwise they are only recommended for use from 3-6 months upwards depending on how well your child sits up and how reclined the seat unit goes. Bear in mind, some have a kind of hinged recline so go from L-shape to flat (or _ ) whereas others have a more moulded kind of seat which only tips backwards, so your baby ends up napping in a kind of V shape instead.
Then you'll have the option on some pushchairs to add a carrycot which makes it into a more traditional type of pram as in a completely flat bed on wheels. Some carrycots can only be used on the pram, and some also double up as a moses basket/portable bed for the baby. Typically a carrycot will be usable until 4-6 months depending on the size of your baby. You may find by 4 months they get nosy and want to see out more than the carrycot allows.
Lastly some pushchairs have the additional extra option of adding a car seat. Usually you'll need special adaptors to attach it. This will only be the infant type of car seat with the carry handle which works up until around 12-18 months depending how big your baby is. You'll probably only want to use the car seat in this way until your baby is around 9 months as they get very heavy. It's not really recommended to use the car seat on the pushchair for long periods because the angle is quite upright and not very healthy for newborns, but it is useful for quick trips, or to reduce bulk in the car especially if your pushchair's seat unit can be totally removed. This kind of car seat can always be fitted into your car using the car's seatbelt, but some of them also come with the option of an isofix base which means you can just click the car seat in and out of the car without any fiddling.
Jargon buster:
Travel system = will include a car seat
2 in 1 = has a toddler seat plus either a carrycot OR the seat converts to parent facing lie flat
Pram = definitely has a carrycot
Stroller/buggy = likely lightweight and suitable for older child only
IMO - look at the following brands:
Joie
Britax
Cybex (not their top end ones)
Mothercare
Mamas and Papas
Mountain Buggy
Cosatto (only if you're into bright colours and patterns - not much to differentiate otherwise)
You can also get some really good deals on the more expensive makes, Quinny, Bugaboo, Uppababy etc if you're willing to look second hand.