24 Legacy is a new show set in the same 'universe' as the original Fox series - basically Kiefer Sutherland won't be back but the events are all going to be pretty familar.
There's a new lead in the form of Corey Hawkins and the show is still casting up, with the annoucement of Jimmy Smits as a major part of the ongoing series. He's set to play a US senator who is married to Miranda Otto who is playing the former head of the CTU. Smits will be up for a presidential election as well, so that's likely to tie into some of the events of the series.
Legacy will take place over 12 episodes and will likely cover the same 25 hour span at the original series. It's not know if there will be real-time elements but it seems pretty likely as its pretty much the defining element of the show, especially with Sutherland absent this time around. It also sounds like these events might be taking place in the future, or at least at a different period to the wacky timeline of 24.
Expect more news on Legacy in 2016.