This means that they contain an array that refers to all the items of a particular type. For example, suppose that you have two images on your page. In that case, document.images is a two-element array. You use collections to modify a group of elements all at once or when elements aren't named. By using collections rather than individually named elements, you can write scripts that work regardless of the number of elements that are on the page.
Collections provide a very powerful way to group items on a page.
You can give the same name to multiple elements and then get a collection for all the elements of that name. For example, if you want to manipulate four different objects at the same time, you could give them the same name, get the collection of those objects, and programmatically enumerate through them. The following is a quick example of using collections. This page contains a set of images. Each time that you click on the page, the images are shrunk.