A bondage hood (also called a gimp mask) is a fetishistic hood. It may be made from rubber, PVC, or leather. It is designed to cover the head completely, and thus has an opening at the back to extend the neck hole for adorning - which may be done up with a zip or cord. Other openings include two eye holes (sometimes these can be covered with a blindfold), optional nostril openings, and a mouth opening (which is often closed with a zip or a gag).
An isolation hood is a more extreme version of a bondage hood; it is design to disorientate the wearer by keeping them in total darkness and making it very difficult to hear anything. Most isolation hoods also incorporate gags to enhance the sensation of loss of control and helplessness felt by the victim. Note that this is potentially extremely dangerous, as blocking all access to air will result in rapid asphyxiation. The safety warnings regarding gags also apply. It is notably used in BDSM, its function being to objectify the wearer as well as providing sensory deprivation, and thus reduce them to the status of a sexual toy, rather than a sexual partner, as part of a scene.
A decorative hood may be worn by a dominant in order to dehumanise them in a different way - to make them appear more distant and impersonal. This can make the submissive feel even more submissive or humiliated by denying them the face-to-face contact that would otherwise occur. Wearing of a hood by a dominant may also allow them to get "into role" more deeply and make it easier to assume the intended attitude towards the submissive.
Hoods can also serve the function of disguising identity when desired, for example at play parties.