Shit is a colloquial synonym for faeces. It is very similar to the word crap, but ruder; in the UK, children can say that something is 'crap' in front of their parents without fear of a clip around the ear, whereas 'shit' is verboten. A single stool, or a single large mass of faeces, is 'a shit'.
Several stools are either 'lots of shit' or 'shits' - the latter is particularly used when referring to a group of people you do not like, such as the middle classes, women, or journalists. 'X are such ignorant shits' is a mighty fine declaration. Just as 'flip' and 'flaming' are nice ways of saying 'Fuck' and 'Fucking', Sex Child Porn the words 'shucks', 'sheesh' and 'shoot' (in the 'J W Pepper' sense) are expurgated version of 'shit'.

Rural and Northern areas of the UK also use the word 'shite' (as in 'bag'o'shite'), which is much less offensive.