A sex doll (also called love doll or blow up doll) is a sex toy. It has the size of a sexual partner, and should help with masturbation.
Basically, there are two variations:
Inflatable dolls; they can be filled with air before use
Those modelled after mannequins; more expensive, but more realistic.
The openings in sex dolls are often fitted with vibrators. Prices for sex dolls range from about ten euros for an inflatable one, to several thousand euros for models with realistic skin.