MouseOver

A Mouseover (or rollover) "happens" when your mouse scrolls over text or images on a webpage, and something else pops up (such as a link, a menu, or an ad). You do not have to click on the text or image for the mouseover to display; it simply appears when your cursor moves over it and (usually) disappears when you move away (though often pop-up windows require you to click and close them manually). A Mouseover is created

The text or image you rollover to activate a mouseover could be a word, a hyperlink, a menu, an icon, or an image on a webpage. The mouseover might show you a drop down menu or submenu, a hyperlink, a brief definition of a word, a pop up window, and/or an advertisement.

Mouseovers can be used to make a webpage more interactive (and hopefully inviting web visitors to stay awhile). They are used frequently in online form applications and submissions. Mouseovers also help organize information (such as presenting navigation options into easily-scannable drop down menus).
A Mouseover is inserted into 2 spots on the HTML document of a webpage, including the exact place in the body text where the Mouseover "event" is to take place. Webmasters and web designers create Mouseovers using JavaScript or CSS (Cascading Style Sheets).