A User Account is a method of keeping track of users on a given system. These accounts are frequently used to allow an individual (or group) permission to use a certain number of resources provided by the system.
On the internet these accounts often offer the user convenience, through profiles, bookmarks and other navigational tools, as well as allows the user to participate in features of a given website such as commenting on forums and various other social features.
In the case of websites like Facebook and Google these accounts are often Data Collective, which helps the host site find a user's Interests and measure interactions to better target Advertising.