The certified aircraft technician/engineer is the person qualified and authorized, according to his/her certificate approvals, to take care about aircraft (airplanes and helicopters) maintenance.
Taking care about aircraft maintenance means to do the required/mandatory maintenance and to sign the Certificate of Release to Service (CRS). CRS has to be signed by a certified aircraft technician/engineer before any flight at the completion of any maintenance.
The certified aircraft technician/engineer is working into an approved/certified company, Aircraft Maintenance Organisation (AMO) or Part-145.
Certification is divided into categories (A and B) and sub-categories (according to the aircraft type, the engine system, etc).