Venom invented the term 'black metal' and pioneered the style, even though it wasn't to become widely popular for almost a decade. Legend has it the band started as a joke: the first two albums, classics though they are, are sloppy and badly-produced, although this is probably not actually the case as Venom had existed for more than a year before charismatic frontman Chronos joined. Venom caught on, however, gaining a massive following that carried them for four albums before cracks appeared. The style changed for the later recordings toward a more traditional thrash/power style, and compared to the early album when they couldn't really play, most of these are pretty wanting. Venom's albums since they reformed in the mid-90s range from poor ('Cast in Stone') to amazing ('Resurrection'). The first four albums are recommended, of course, as is 'Resurrection', which saw them adding a late 90s thrash aspect to their sound. 'Metal Black' is a misguided attempt to go back to their orginal sound.
There are a ton of Venom reissues and compilations around; the ones included below are probably the most comprehensive available as most feature pretty much the same tracks.