Now, I am an expert in 'prams' or whatever you want to call them, (not in the sense 'I've had 10 prams so that makes me an expert'- my entire career to date has involved baby equipment and in particular, prams/pushchairs etc,) so do not argue with me! 
There are only 3 categories. Pram, pushchair, stroller.
Pram(perambulator) has to have a carrycot to be classed as a pram. when a carrycot is removed/converted it may well turn into a 'pushchair'
Anything else is a pushchair other than an umbrella fold style which is a stroller.
A travel system can fall into all 3 of these categories, as it just means it is car seat compatible (either included or sold separately)
a 'Buggy' is a non official slang word for any of the above, and I despise the word.
I also despise people using the term 'pram' for any of the above. tends to be people of the chav variety It only applies to a proper pram, not a shitty little stroller.
it is confusing as you could buy a pushchair, that is car seat and carry cot compatible, making it either a pram/pushchair/travel system depending what mode you are using it in.