The DNC is THE Democratic party, and it consists of members from all over the country, representatives of areas, just like Congress, except this group represents other Democrats. It is the official body of the Democratic party.
The DSCC is the group run by the Dems in the Senate and funded by their various contributors. The goal of the DSCC is to recruit, fund, and elect Democrats to the Senate.
The DCCC is the group run the Dems in the House and funded by their various contributors. Their goal is to recruit, fund, and elect Democrats to the House.
The Senate and House members who run the DSCC and DCCC are usually in the leadership of the party for that house of congress, and they usually are looking to collect IOUs for future races.
There is no doubt that guys like Emanuel are looking to add members who OWE him. Same as the Tom DeLay template for action.
The DLC is only a lobby/think tank. It's not really a think tank, either, although it floats position papers. Some have said Al From, its founder and leader, is a Democrat In Name Only, or a DINO. While that may be accurate, he's still backing Dems for president, and our last few presidential candidates have been loosely associated with the DLC.
I don't personally consider a pol's membership in the DLC to mean anything. To me, it's like them joining the Rotary Club. Some Dems feel threatened by the presence of the DLC, as reflected in yesterday's cartoon.