Refers to either text or buttons, they encourage or illicit a response in the user prompting a desired outcome. This outcome can range from buying a product to signing a newspaper or whatever the webmaster had in mind. Like all designs, there are good ones and there are ineffective ones, take for example NetFlix's main webpage. The bold red button encourages and reinforces the user to sign up for a month. The same format holds for Prezi, a major online powerpoint creator. If used correctly, Call To Action media will greatly improve communication with the user and their navigation of your website.
At times there are multiple URLs that link to the same content. For example on a specific e-commerce website, there could be multiple links spread out throughout the entire website that would refer to a products page. This type of linkage is confusing to a search engine, forcing it to make a wild guess on which link is the correct one. This is where the canonical URL tag comes in, it tells the search engine which link is the true link and focuses all resources onto that link.