The EUSO-SPB (Extreme Universe Space Observatory on a Super Pressure Balloon) is a balloon borne UV camera. It is dedicated to the observation of fluorescent light produced by UHECR induced air showers from the stratosphere. EUSO-SPB was launched on the 24th of April 2017, 22:50 (UTC) and lasted for 12 days 12 hours at a nominal altitude of 33.5 km. The main instrument consists of a two Fresnel lens system and a fast UV camera recording images with a time resolution of 2.5 $\mathrm{\mu}$s. The camera consists of a pixelized focal surface composed of 36 Multi-Anode photo-multipliers (MAPMT). The optical system includes two Fresnel lenses of 1~m diameter with a wide field of view. In addition of the main UV camera, EUSO-SPB flew with two auxiliary systems, an IR camera and a SiPM UV camera. In this paper, the different subsystems of the EUSO-SPB instrument will be described: the optical system, the focal surface with its readout electronic, the different subsystems controlling the data acquisition and the auxiliary system.