It all depends on the version of Parasite.
One post crisis version has a power like speed steal that can absorb momentum as well as absorbing a person's memory and intellect, DNA to literally replicate them and also had shape shifting abilities. Nu52 could steal speed force, other versions could also steal life force as well as powers.
Some versions could absorb powers from a reasonable proximity (the the one that fought Firestorm and Firehawk could from a 100 meters away) and others could absorb or even lend powers from and to anyone they could see (one such version stole and lent Airwave's power to his enemy from the side of the battle and innthe same story stile Supermans strength as he threw a punch then gave him back just enough invulnerability to take the punch he threw back to make the fight last.