When a party is surprised, the individuals on the other side can perform a full round of melee attacks each surprise segment (DMG page 62).
If and individual in the surprising party has a missile weapon readied beforehand, a full round of missile attacks can be made at three times the normal rate—otherwise a full round of missile attacks at normal rate—each surprise segment.
When moving, however, only one segment of movement can be carried out (see PHB page 102).
Magic-users and clerics in a surprising party can cast a spell with a casting time of 1 segment or begin casting a spell with a longer casting time.
Spell casters seem to get shafted by these rules. Furthermore, the advantage given to readied missile weapons seems overpowered. If I’m in a party that surprises a group of monsters, and I have composite bow (rate of fire is 2) readied with an arrow, that means I get six missile attacks in the surprise segment! My potential damage range against a small or medium creature that segment goes from 2-12 to 6-36!