They appear as poison-green humanoid demons wielding huge demonic blades. They have a powerful Inferno attack along with their fast melee attack. Like their name implies, the have a chance to do Poison damage in their attacks. The same applies to a very similar Pit Lord variant.
The Infector of Souls is a super unique Venom Lord who is found in the Chaos Sanctuary as one of Diablo's favored generals when you open one of the south-eastern seals to release Diablo. Baal in the Worldstone Keep summons Venom Lords with a boss.