Squalor is the state of being filthy and miserable to live in — and the word almost always carries the reason: nobody had the money, or nobody cared enough, to keep the place decent. A room can be messy without being squalid; squalor is dirt plus neglect plus, usually, poverty. Think of broken windows nobody fixes, damp walls, rubbish that has been there for months, too many people in too small a space. It is an uncountable noun, so no 'a squalor' and no 'squalors': 'living in squalor', 'the squalor of the camp'. It is a fairly formal, written word — journalists and social workers use it, and it appears a lot in descriptions of slums and prisons. It carries strong judgement, and often the judgement is aimed at whoever allowed the conditions rather than at the people living in them. Writers sometimes pair it with something rich for contrast — 'squalor and splendour side by side' — which is why you meet it in travel writing about big cities. The adjective is squalid.