Hi David,
There are two different products:
ArcGIS Runtime SDK for WPF (retired as of July 2016, last release v10.2.5)
ArcGIS Runtime SDK for .NET (current release v100.0, previous release v10.2.7)
The '.NET SDK' is essentially an evolution of the old 'WPF SDK'. It still contains an API for WPF because it's still a major development platform. But over time we have had to stop supporting some platforms as they have been deprecated/retired or are no longer promoted (Silverlight, Windows Phone 7/8, Store 8.1, Phone 8.1). We had a significant version number change from v10.2.7 to v100.0 to indicate the extent of the change in terms of internal architecture, public API surface and the SDK product. The v10.2.7 release is currently in the Extended Support phase, which continues right through to November 2018 to allow time to move to v100.0 (Esri Support 10.2.7).
If you're just starting your development now you should definitely use v100.0.
Doc for the current/latest release: https://developers.arcgis.com/net/
Doc for the previous version: https://developers.arcgis.com/net/10-2/
Thanks for the feedback on navigating versions on these forums, I'll pass that on.
Cheers
Mike