Starting with the Nexus One in 2010, Google has maintained the Nexus line of phones, tablets, and media players. In recent history, the Nexus line became known for (relatively) inexpensive devices with timely Android updates. But it looks like the Nexus 6P and 5X will be the final devices to bear the Nexus name.
Google confirmed to us at their Pixel event today that they have no plans to develop future Nexus products. Presumably, all of Google's hardware efforts moving forward will be under the Pixel banner, which already is populated by phones, tablets, and Chromebooks.
While it might be easy to consider the Nexus lineup as the best Android devices hands-down, it did have difficulties. Besides possibly the Nexus 7 tablet, it's hard to say that any Nexus device was ever popular with the average consumer. Most devices relieved little-to-no advertising, and with different manufacturers with every generation, some devices were built better than others. The Nexus line also had difficulty with carrier adoption, often only one or two US carriers would sell the newest device.
That being said, the evolution of Nexus devices coincided directly with Android as an operating system. Nexus was Google's playground to develop Android with, and hopefully Pixel will continue that legacy.