As an act between two individuals, grinding presents rubbing hips together while doing it in a rhythmic motion. Although grinding is very similar to dry humping, in the form of activity, it is a far less sexual one. Grinding, unlike dry humping, most often happens on dance floors between couples.
There are many different ways in which the grinding can be done. The most common method of executing grinding is by achieving a spooning position in which the front of the one partner is pressed against the back of the other partner. Another way to grind is when the couple faces each other while rubbing hips together.
When it comes to sexual grinding, the activity can be performed as foreplay or by people who abstain from sex but are not opposed to activities that involve heavy petting. The general term, however, indicates the individuals are fully clothed.
Outside of this context, grinding can be a descriptive term for the movement of the woman's hips while she is on the top of her partner during sexual intercourse. In this case, grinding describes the deep movement the woman's hips are doing while the couple is having sex so it represents a synonym of riding.