Yup, like Pumbaa said, it's basically what it sounds like... a place for you to record all of the thoughts and ideas that you have for your wedding, or just describe what you're doing. It's a great way to give inspiration to other couples and also by reading others you can get some really good ideas. It's helped me a lot to post about "hey, I can't decide whether my bridesmaids should wear black shoes or silver, what do you think looks better with this color dress?" You know, stuff no one else really cares about except other people getting married!
And yeah, when you need to vent or rant, it's a good place for that too, and you'll be talking you other people who understand exactly what you're going through.