Sloes are small wild plums - sort of blue/black in colour. You wash and prick each sloe with a thorn from the bush, put them into a bottle with some sugar and some almonds, top with gin then leave it for a few months. Strain the liquid out into another bottle and it is ready for drinking. It does not taste ginny as it can be pretty sweet.
There are lots of sloe gin recipes but this is the one we use and it is lovely!
Sloes are Blackthorn rather than Hawthorn berries. An alternative is Damson Gin - I harvested our damsons today so will be making this for the first time this week... you prick the fruit all over then mix with granulated sugar and gin. It doesn't taste of gin (well, shop-bought it doesn't), tastes very sweet and almost port-like.