Start the library module, select an image and then press C on your keyboard.
Lightroom then shows the selected image (Select) and the right image next to it in the filmstrip (Candidate). With your arrow keys you can 'browse' through your filmstrip to select another picture.
You can change the selected image by clicking on another picture in the filmstrip
Instead of using the arrow keys of your keyboard you can use the arrows direct under the candidate to select another candidate.
With the zoom option you can zoom in on both pictures. This can be very helpful to see if one picture is for example sharper then the other picture.
Under the Candidate are 2 more buttons, one to swap the Selected and the Candidate and one to make the Candidate the Select.
The compare view in combination with rating can be very useful to select the image you want to develop. To quit the Compare View just click on C of your keyboard.
I always use this when doing my post processing, it's part of the workflow.