Med Packs are consumables which restore a certain amount of health. They are single-use and can be purchased from medical vendors.
There are three kinds of Med Packs:
Basic Med Pack -- healing over 5 seconds with a 5 second cooldown
Standard Med Pack -- roughly one and a half times the amount of healing of a basic pack at twice the credit cost over 5 seconds with a 10 second cooldown
Advanced Med Pack -- roughly twice the amount of healing of a basic pack at four times the credit cost over 5 seconds with a 15 second cooldown
Note that using any Med Pack creates a cooldown on all Med Packs, meaning you cannot use a Basic Med Pack and immediately follow it with an Advanced Med Pack.