Google Chrome is a Webkit-based browser. It is the non-open-source version of Chromium and differs in having several proprietary add-ons, such as custom flash and PDF plugins and crash and user metric reporting (not enabled by default).
This tag is for Google's browser, Chrome. For questions on Chromium read and use chromium.
Chrome is not available in the default Ubuntu repositories, but you can install it by using Google's PPA. Ubuntu ships Chromium, the open source project that Chrome is built from. See this for a detailed list of differences between Google Chrome and Chromium.
Since version 28, all Chromium based browsers use a forked version of Webkit, called Blink.