There will be a modding tool, called the Creation Kit.
They are hoping to have it ready for the public on day 1, but they are still not sure on that part.
The Creation Kit uses NIF Format, and the reason behind that is as Todd explains, the modders knows the format. (They care about us!)
It uses a new Scripting Language, but how new it is, is still uncertain. (i.e, will we see the same commands, or something entirely new?)
As for how it looks, it's very much like the Construction Set, just very much improved.
Mods for Consoles will, according to Todd Howard, most likely not make it for Skyrim, but he is hoping that it'll make it for the next game. (TES VI confirmed?^^)