There are usually 2 types of data recovery (this is for hard disk reference. other devices i am not sure e.g flash drive, memory card, etc...)
1. software (cheap) - use a particular software to recover you data. usually takes a day or two depending on size
2 hardware (very expensive) - meaning the circuit board, motor, read write head or some physical part of the hard disk is spoilt. The disc is actually very hard. Therefore, data stored is actually quite safe. It is transferred to another hard disk and files are then being recovered.