When "describing" the work of another thinker, one quickly faces the problem of how much detail to include (see my "A Guide for Writing Philosophy Papers,"). Too much detail and you will not be able to see the forest for the trees, too little, and you will have only vague generalizations. Also too much detail can result in your not leaving sufficient time for the most important arguments. These problems can be easily solved by having a focus.
What is a focus? A focus is a smaller part of an article or chapter selected for more detailed attention. So actually, to solve all the problems listed above, you will need both an overview and a focus. The overview should enable you and your readers to "see the forest", while the focus will pick out two or three trees for detailed observation. The overview is thus also very important. It provides the context. This is in keeping with the metaphor of focusing. When one focuses a telescope or microscope on some object to bring out its details, one often loses sight of the surrounding context.
How small should a focus be? This can vary considerably. A focus could be a single paragraph, which can then be described in great detail. The significance of even individual words used in an argument can be debated. A focus could be as large as maybe three or four pages.
On what should one focus? You should usually focus on the strongest argument you find in the article or chapter. Sometimes you might wish to discuss some issue other than the main issue discussed in the chapter or article, but find the writing still has relevance to your issue; in such a case you would usually choose to focus on the strongest argument relevant to your issue. Picking the strongest argument is not a hard and fast rule. If you choose to argue against a piece of writing then it is usually more interesting to see if you can make headway against the best of the arguments. But it can be interesting to see whether secondary arguments can be undermined, especially if there is already broad agreement about the strength or weakness of the strongest argument. On the other hand, if you want to argue in favour of a piece of writing, then choosing a weaker argument as a focus in order to defend or refurbish it later, in your analysis, can be very appropriate.
Is the focus part of the "description" or part of the "analysis"? Since the focus describes a smaller part of an article or chapter in greater detail, one might think it is part of the description. The focus does however, select from the larger piece of writing and the act of selecting is not descriptive but interpretive. Certainly, describing always involves some interpretation, even so, I would suggest that the interpretive element plays a larger role in the focus than it should in the overview. Thus the focus already has one foot in "analysis" metaphorically speaking.