Hold onto your hats, it’s about to get wild. Just like it says on the tin, sour beers are characterised by their sourness. They are made using wild yeasts, which used to just be present in beer production in general back in the day, meaning most beers had a touch of sourness to them. Over the course of time, the environments where beer was produced allowed brewers to guard the ingredients against the wild yeast, though some producers intentionally include them to create this particular style.
One of the main styles of sour beers is the Belgian Lambic, which is made using a combination of both wheat and barley, with the wort cooled in open shallow vats, letting the wild yeasts do their thing. Plenty of hops are also used in Lambic beers, but these aren't used for flavour like most beers - brewers use the hops for their preserving qualities for their beer's long maturations, hence why they use old hops which have lost their bittering qualities. Lambic beers are well aged in oak casks before bottling.