As Bryan Singer mentioned in his announcement, Quicksilver (a.k.a. Pietro Maximoff) was a part of the X-Men universe before becoming an Avenger with his sister Scarlet Witch (a.k.a. Wanda Maximoff). Quicksilver is the son of Magneto, played by both Michael Fassbender and Ian McKellen in the X-Men universe, using his power of super-human speed to do evil for the Brotherhood of Evil Mutants. No specifics were given as to how this character fits into the sequel.
Bryan Singer is directing from a screenplay by Simon Kinberg, adapted from a 1980s comic book arc by Chris Claremont and John Byrne. The story is set in an alternate future where mutants are hunted down by Sentinels, following the murder of an important Senator.
Production is currently under way in Montreal. We'll keep you posted regarding any further updates as shooting continues in Canada.