So I guess what I am hearing/reading here is that Mitch Kupchak is one heck of a general manager, and/or there are so many loopholes in the NBA player/owner agreement that if your smart, there is no salary cap. I just don't get it.
The article that Roy linked to is the most comprehensive, understandable, and informative explanation out there, in my opinion.
The general idea is that the NBA's cap is not a hard cap, but is a soft cap, which means that it can be exceeded following the rules of certain cap exceptions. When a team is over the cap, they can not sign free agents unless an exception is used.
The Celtics are over the cap. They have the bi-annual exception (which I believe is in the $1.5-$2 million range) and the league minimum exception with which to sign players. They can also make trades. Beyond those exceptions, they can not sign players.