Overview
Dips are common to many partner dance styles (Tango, Lindy Hop, Salsa, Ballroom dances).
Characteristics of a dip include:
weight-sharing
dancers' centres go lower than when standing
Some dance styles distinguish between sits, dips, and drops; as a general rule, the difference is the degree to which the base (usually the lead) ends up responsible for the mass and balance of the flyer (usually the follow). Where a distinction is made, it's usually a continuum from least to most responsibility, with sits being the least, drops the most, and dips in between.
There are infinite varieties of dips. Following are some descriptions of some dips; these may also be known by different names in different dance styles, and in different groups within the same dance style.
From Wikipedia (CC BY-SA 4.0).