The first of the Nexus line was meant to shake up the way we buy and use our phones
The Nexus One was the original "Google Phone," manufactured by HTC with "Pure Google" as the sole driver. From the software — no carrier or manufacturer customizations — to being sold online only, directly from Google, and SIM-unlocked at that. It launched on Jan. 5, 2010, as a GSM-only model.
Commercially, the Nexus One was a flop, and Google's Nexus One store shut down. The idea of revolutionizing the way we buy phones — at full price and without actually getting to try them first — never really materialized. But Google's path was clear: Nexus was a thing. And every year (more or less) since Google has released a new Nexus, a sort of developer/reference device that certainly has grown into a fully capable commercial device.
See also: Nexus S, Galaxy Nexus, Nexus 4, Nexus 5, and the Nexus 7 and Nexus 10 tablets
We don't know how much longer the Nexus line will continue. There are rumors that it'll be replaced by some sort of "Android Silver" program. But we'll always remember our first.