Jump packs allow models to move 12" per turn in the movement phase, and ignore difficult terrain as they move. Characters leading a Command squad or who are part of a unit can only use a jump pack if all models in the unit have jump packs.
Space Marine models who are wearing jump packs can be dropped from low-flying Thunderhawk gunships, using their jump packs to swoop down on to the battlefield. To represent this tactic they can set up using the Deep Strike rules, but only if the mission allows for Deep Strike to be used. If the mission does not allow troops to use Deep Strike then the model must set up normally with the rest of the army.