A needle roller bearing is a bearing which uses small cylindrical rollers.
They are used to reduce friction of a rotating surface.
Needle bearings have a large surface area that is in contact with the bearing outer surfaces compared to ball bearings. Additionally there is less added clearance (difference between the diameter of the shaft and the diameter of the bearing) so they are much more compact. The typical structure consists of an inner race (or sometimes merely a shaft), a needle cage which orients and contains the needle rollers, the needle rollers themselves, and an outer race.
Radial needle bearings are cylindrical and use rollers parallel to the axis of the shaft. Thrust needle bearings are flat and use a radial pattern of needles.