I have been asked several times “when do you use <article> and when do you use <section> in HTML5?” I use an analogy that I figured I would share, as it seems to help developers make sense of the two elements.
Think of a newspaper. The paper comes in sections. You have the sports section, real estate section, maybe home & garden section, etc. Each of those sections, in turn, has articles in it. And, some of those articles are divided into sections themselves.
In other words, you can have parent <section>s with nested <article>s that in turn have one or many <section>s. Not all pages documents need these, but it is perfectly acceptable and correct to nest this way.
I hope that helps explain it.