What's the difference between hypsography and topography?

Hypsography concerns the land's elevation, altitude or height above sea-level or some other reference surface. (Hypso is derived from the Greek Ύψος for height.)

Topography concerns physical and cultural features of the land and so includes hypsography, hydrology, the built environment, major boundaries, communication channels, etc. (Topo is derived from the Greek Τόπος for place.)

Note, however, that North Americans often (incorrectly) use "topography" when they mean only "elevation" (or "hypsography").

Hypsography can be used to describe the elevation of the land as it's related to other areas (ie, mean sea level).

Topography can be used to describe the features on that elevated land and also how they are related (ie, their spatial relationship) to each other.