The headset is the main body of the VR unit. It contains the optics, lenses, LED or OLED screens. Headsets have a strap connected to them. There are various designs, for example, halo,  but nearly all are constructed of some kind of strap and the rigid part that cups the back of the users head.

Also connected to the headset will be the in-built headphones. Plugging in optional external headphones is also a possibility with most headsets as they nearly all have 3mm audio jack connectors.

Some Headset have Mechanical IPD Adjustment
Mechanical IPD Adjustment

Some Headsets will have a mechanical IPD adjustment. On the HP Reverb G2, this is located as a slider on the underside of the unit.