WebDriver Collections are essentially a number of web page elements captured in an array. WebDriver provides us with the ability to capture multiple elements in one array object. The objects gathered in the array usually contain something which is common amongst them all, for instance tag or class name.
WebDriver Collections Video Tutorial
When capturing multiple elements as part of a collection we can loop through each element and act on it. There are multiple occasions where it makes sense to capture multiple elements as part of your tests. An example of this may be to get all the text entry fields on a form and fill in all fields in one go.