Crossovers between Arrow and The Flash can be so much fun!
From the look of the first photo of the big event, leaked on Twitter by executive producer Marc Guggenheim, it will be a launching pad for their upcoming series DC's Legends of Tomorrow.

I'm using my best guess to say we're looking at a photo from the Arrow portion of the crossover, but hey, the entire Arrow team could very well be crossing over to The Flash, as well.
Airing on Tues., Dec. 1 and Wed., Dec., 2, the two part episode is just that. A genuine two-part story that will flow through both hours and not be self contained as we have understood crossovers to be in the past.
In addition to the characters we know so well, his photo features Ciara Renee as Kendra Saunders/Hawkgirl played (who we have recently gotten to know a little bit in Central City) and Falk Hentschel as Carter Hall/Hawkman, who we have yet to meet.
Whether there will be other Legends characters involved in the crossover? We'll just have to wait and see!
In the meantime, you can tune into The Flash Season 2 Episode 6 tonight and Arrow Season 4 Episode 6 tomorrow night with full reviews shortly after air!