Cracked.com can be a little annoying with how they sound like angry annoyed jerks at life in most of their articles, but I still enjoy reading them because they are quite informative and fun to read, unlike most other sites regarding similar topics.

As for your last question about why people type "CRACKED!" in the replies to YouTube videos linked by the site, it's like tagging. I think. This is the same for most other popular sites that link to YouTube videos. You'll notice replies like "Like this if _____.com took you here!" and whatnot.