A keyword or phrase given to a document, image, record, etc. online that associates the given piece of information with that keyword or phrase.  Tagging information causes it show up when that keyword or phrase is searched.

A phrase commonly used to explain where data is stored for programs like Office365, Google Docs, and other web-based services.  The Cloud is a collection of servers based around the world either owned, or leased by, companies in order to host user information.  These servers are connected to the internet allowing remote access to data from around the globe and resulting in the idea that user's information is In the Cloud.