Baby carriage or Perambulator shortened to Pram is a cradle on wheels. Babies lie in them as you push the pram around.
Strollers, on the other hand, are chairs on wheels. Your baby sits in them upright as you give them a ride. Pushchair or buggy are other names for strollers.
With the variations that allow the baby to either lie down or sit upright, it may be difficult to tell the difference. Sometimes Prams and Strollers are used interchangeably.
Prams are for newborns who are yet unable to sit. They usually have a soft flat bottom that allows the baby to sleep. It also has a half-canopy to keep them covered as they sleep.
Strollers are for infants who have learned to sit. They have safety harnesses and safety belts to keep the babies secure.
Just like a bed is bulkier and heavier than a chair, prams are bulkier and often heavier than strollers which make prams more difficult to transport.
Prams are often are taller and further away from the ground while strollers ride lower.
There are of course hybrids.