X-Men: First Class is the second prequel to the X-Men trilogy, going back to the beginning of the friendship between Charles Xavier and Erik Lehnsherr (aka Professor X and Magneto). It's a serviceable film, but might be confusing to the uninitiated, especially since it doesn't quite keep to continuity with the other films. Like the other X-films, there are plenty of second-string mutants to keep track of. But the complex relationship between Xavier and Magneto has always been the core of the franchise (much like Peter Parker's mundane problems in Spider-Man, or Bruce Wayne's angst in Batman), and First Class's strength is that it focuses on that.
The main action occurs in the early 1960s, in events leading up to the Cuban Missile Crisis. This is perfectly logical, given Magneto's childhood as a concentration camp survivor, and it dovetails neatly with the (real-world) historical debut of the Xavier and company in 1963's X-Men #1. It would have been nice to be a little more accurate in the details, however -- for example, the Soviet Union is always referred to as Russia in the movie, even when referred to in dopey subtitles like "Russian Military Retreat."
First Class is very much of the present time in its setting, however, since there is a spate of 1960s-themed shows and movies lately, viz. Mad Men, The Playboy Club, Pan Am, and so on. It channels the spy-fi aesthetic of the early Bond films well without veering too much into Austin Powers-style farce, though it's not quite as sophisticated-and-at-the-same-time-irrepressibly-1960s as The Prisoner. Then again, what is?
What's funny about all this sixties throwback stuff is that in other ways, the Blu-Ray of X-Men: First Class is right on the technological cutting edge: the bonus features are typical of current extra "value" that costs nothing for the studio and only requires you to consent to tracking via the corporate equivalent of Cerebro.