I don't understand how Thief's Venom skills work...

I understand these skills have other condition effects and one even has a poison effect. But, what is Venom? I still have no idea.

Example: Venom Duration: 30 seconds. http://i37.photobucket.com/albums/e71/muse100/f4099a6a.png

Does it damage a player's health? How much damage? Does it do more damage in one skill as opposed to a different Venom skill? Does it lower an opponent's stats? Explain please? Is it removable by opponents in PvP?

I didn't find anything on it in guildwars2 wiki and I tried doing a search for it here on reddit, but I didn't exactly find what I was looking for. Please help me.