The Modern Masters set is a 229-card expansion that contains reprints of Modern-legal cards from Eighth Edition through Alara Reborn. The set will not be Standard legal.
The size and structure of the set is similar to any large set release. There's the standard recipe 101 commons, 60 uncommons, 53 rares, and 15 mythic rares. Some cards changed rarities from their original printings either because (a) we needed some mythics, and a lot of these cards are originally from sets which predate mythics, or (b) we wanted to balance the limited environment. There are no basic lands in the set; instead, the fifteenth card in each pack is a foil. That's right—a foil in every pack!