Testing UI differences based on predefined set of images.
The image that is taken on a known state, validated by a person, and later used to find visual regressions.
A regression image is created whenever the automatic test has found a mismatch between the baseline image and the current state of the site's page.
Selenium is the industry standard for automating interactions with browsers such as Chrome, Firefox and Internet Explorer.
BrowserStack / Sauce Labs
Paid services that allow provisioning a virtual machine with a pre-defined platform and browser. These services remove the need of maintaing your own Selenium could.
This Yeoman generator, also known as "yo shoov", scaffolds all the needed files for quickly starting visual monitoring of any website.
An open source project that facilitates maintaining visual regression tests, and allows one click configuration and execution of visual and functional tests on any dev/ live website.