Sven Co-op is, as you've guessed, a Co-operative mod. Over the years it has seen a vast number of improvements; new maps that get better with every release, fantastic weaponry, and of course a large array of monsters and nasty Humans to swipe the floors clean with you. Most maps have objectives, and most maps have a vast army of monsters between you & your goal. Now it seems like your average Vanilla HL singleplayer so far, but let me tell you it's anything but that. The monsters are tougher, they work together better & it is very easy to get overwhelmed. Suddenly you see where the Co-op aspects work; to stand a chance you have to work in groups, covering fire & healing each other when needed. Wherether it works well depends on who you're playing with, but if they're good than you'll notice the difference. It's an amazing feeling to work together fighting through a map - although sometimes (although this really depends on the Server or the people you play with) gameplay feels a bit random as people run to the action instead of moving together.