A literature review can be just a simple summary of the sources, but it usually has an organizational pattern and combines both summary and synthesis. A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information. It might give a new interpretation of old material or combine new with old interpretations. Or it might trace the intellectual progression of the field, including major debates. And depending on the situation, the literature review may evaluate the sources and advise the reader on the most pertinent or relevant.
How is a literature review different from an academic research paper?
The main focus of an academic research paper is to develop a new argument, and a research paper will contain a literature review as one of its parts. In a research paper, you use the literature as a foundation and as support for a new insight that you contribute. The focus of a literature review, however, is to summarize and synthesize the arguments and ideas of others without adding new contributions.
How do I know when I can stop?
Literature reviews can be tricky because you don't want to stop before you've found everything relevant to your topic. There are a couple of guidelines for knowing when to stop looking for materials.
If you have done steps 1.1-1.3 (below), when you start to see the same articles over again, then you have done your due diligence and can consider your lit review complete. That isn't to say an article might not slip through, but if you have done the steps below, then the chances of a really important article slipping past you is pretty slim.
Searched all relevant databases, using a variety of keywords and subject headings
Mined article bibliographies for their cited references
Think of the assignment timeline. If you are writing your PhD thesis you can spend more time doing a comprehensive lit review than if you only have a few weeks until an assignment is due. At some point you need to stop.