An intelligence officer is a person employed to collect and or analyze information (known as "intelligence") which is of use to the organization.
The word officer is a working title, not a rank. It is used in the same way a "police officer" can be a sergeant, or in the military, where non-commissioned staff may serve as intelligence officers.
Organizations which employ intelligence officers include armed forces, police, and customs agencies. Mainly, though, the term is used by government intelligence agencies like MI5 and MI6.