Google has released an open SDK for integrating their Google Assistant service to your devices. The primary platform for this SDK is the Raspberry Pi, and it installs there really easily.
Google Assistant is a service by Google that offers voice recognition and enables you to run custom commands based on this input. Comparable services are Apple's Siri, Microsoft's Cortana and Google's Google Now. With the SDK you can run this software on a Raspberry Pi and you can program it to respond to your custom made commands.
Installing the SDK is really easy, Google Developer portal has the instructions. Hardware-wise you will just need a Raspberry Pi, USB microphone, USB speaker (or 3.5mm) and a memory card with Raspbian / NOOBS installed. Optionally you should have a HDMI-enabled monitor. keyboard and mouse available for setup.