A Star is a way of giving credit to a user's entry without using up one of your favourites for the day.
You can see how many Stars have been awarded to a particular entry by looking at the number next to the Star icon, at the foot of the image. Simply click on the star in order to give it.
Unlike favourites, you can give as many Stars as you like. Once you give a Star however, you won't be able to withdraw it, so choose wisely.
You can even give an entry a Star and a Favourite if you feel so inclined!