Hairspray is a sticky liquid you spray from a can onto finished hair so the style stays. You hold the can away from your head, spray a light cloud, and the hair feels a bit stiff and holds its shape — a bun, a wave, or hair that would otherwise go flat or fly up. People use it after they have already cut, dried, or set the hair; it is not shampoo and not the gel you work in with your fingers, though the job (keep hair still) is cousin to gel. It can feel old-fashioned or “stage and school photo,” but plenty of people still use it. Too much makes hair crunchy and white at the surface. The word is everyday, not formal.