I have seen quite a few post on the web mixing up these term, saying things like "Silverlight gets Photosynth", which is not correct.   So ...

Silverlight (http://www.microsoft.com/silverlight/) : cross platform runtime for developing rich internet applications.

Seadragon (http://livelabs.com/seadragon/) : component that allows you to view vast amounts of visual information quickly and with low bandwidth.

Photosynth (http://photosynth.net) : an applicatio which uses the Seadragon platform to allow you to zoom into great detail on photos uploaded to the photosynth website in a 3D environment.

DeepZoom (http://msdn.microsoft.com/en-us/library/cc645050(VS.95).aspx) : is an implementation of the SeaDragon engine inside the Silverlight runtime, exposed as the MultiScaleImage control.

Based on these definitions it would be correct to say "Silverlight gets DeepZoom".