The ElementFinder simply represents a single element of an ElementArrayFinder (and is more like a convenience object). As a result, anything that can be done with an ElementFinder, can also be done using an ElementArrayFinder.
The ElementFinder can be treated as a WebElement for most purposes, in particular, you may perform actions (i.e. click, getText) on them as you would a WebElement. Once an action is performed on an ElementFinder, the latest result from the chain can be accessed using the then method. Unlike a WebElement, an ElementFinder will wait for angular to settle before performing finds or actions.
ElementFinder can be used to build a chain of locators that is used to find an element. An ElementFinder does not actually attempt to find the element until an action is called, which means they can be set up in helper files before the page is available.