How does the Terror watch list work?

Apparently there are One million Americans (and foreigners ?) on the terror watch list among which an 6 year old girl. What does this entail? Does that mean a huge number of American citizens can not board a plane among other things? How do you end up on that watchlist?

I'd assume by having a background dealing with anti-government, anti-establishment, pro-militia, or of course, problems going through security. etc. They purposely just do it for inconvenience and fear mongering. If anyone truly wanted to go somewhere, they could. It just wouldn't be as casual and comfortable for them. And you know how some people get when they aren't comfortable, they get scared and the fear abolishes their will power.

Isuppose if the books you buy are of a sensitive nature, the websites you visit are of a sensitive nature etc......

Big brother is watching us. There are lots of red flags that the Fed throws up. Steer clear of them. I may have just raised one myself.