A rollover is an image which changes in some way when the user's mouse pointer passes over it. It is most often used as an interactive device as part of a site's navigation buttons. Stereotypically the button may appear to light up or squash to indicate a link. If the user chooses not to click the link, the first image is usually restored once the mouse pointer has passed over it. Note that these do not work with most touch screen devices (iPad, iPhone, iPod touch, and probably Android/Blackberry/Windows smartphones and tablets too) as the code is not triggered to show the second image.
To create a rollover you will first need a pair of images in .gif, .jpg or .png format (you can mix and match formats, and gifs can be animated. There are a few examples on my son's first homepage at http://www.jednet.co.uk/jamie
Original Image: Then fill the Original Image box by browsing for the image that you want displayed when the page is first displayed. This original version is also called the 'off', the 'up' or the 'static' state.
Rollover Image: Next browse for your Rollover Image. This is your 'on' or 'over' state.
Preload Rollover Image: Usually you will want to preload your second image to give a good response - otherwise there will be a delay while the browser calls back to the server when it needs the second image state. There is a special case when this may not apply. The rollover image does not have to be a simple unmoving picture - it can be an animated gif. If this is set to loop then it should still be preloaded. If set to play only once or a few times, it will have likely stopped moving long before anyone invokes it, so then DON'T preload. This is also true if it relates closely to the original image and must be seen from frame 1 rather than viewed from any point in the animation.
Dreamweaver can take rollovers to the next level with a function which will create an entire Navigation Bar. This can even automate the production of a table into which the buttons will be ordered. It allows each of the buttons to show a different look for up to FOUR states. In addition to the 'up' and 'over' states used for simple rollovers as detailed above, it allows for extra images to represent 'down' for when the user has their mouse button depressed and the rare 'over while down' when the pointer is over the image after it has been clicked. Complex rollovers where more than one area of the page change on mousing over something are often easier to create in a Web graphics application such as Adobe Photoshop or Fireworks using 'slicing'. You can save from these an html page containing a table, together with all the graphics in their various rollover states. Here's one I created earlier...
The latest trend is to create both the original and over states as a single graphic which is then moved up or down with CSS while just half is visible - easier to keep track of the graphics and guarantees that the over state will load without having to include the cumbersome preload code: www.elated.com/articles/css-rollover-buttons/
Dreamweaver also makes 'Flash' buttons easy to create. These can have sounds attached and can look very good - unless your user lacks the Flash plug-in: in which case your site just lost its navigation (Apple IOS devices - other than computers running Mac OS - don't support Flash, and it is doubtful on other smartphones and TVs etc. It's good practice to offer alternative simple text links at the bottom of the page).