A literary review can take multiple different forms. It could be as short and simple as a rating (like 2 stars out of 10). Or it could be an essay about the original literary work. Typically a review has both objective or factually based information about the work and subjective or opinion based commentary.
In a sense, almost all of the articles here on Literawiki are reviews -- of either literary works or their authors. However, we are often using the term to refer to the more subjective descriptions -- what we liked or didn't, where we would classify the work and who we would recommend read it.
A good review serves multiple purposes. It helps others decide whether this is a book (or other item, such as a movie or essay) that they might enjoy. It is a way of sharing our feelings about the works -- which can also help us find people with similar tastes who might like some of the same things. And a good review teaches us more about literature by helping us to notice factors we might have missed and enriching the reading experience.