What makes a report different than an article?

In general, the audiences and the content are different. You write an article for the public usually... you put in interesting facts, and things that will appeal to readers who aren't that familiar with the subject... has to appeal to a wider audience. The ultimate goal of an article is to sell papers, so you try to make it accurate, but not dry. Most articles will have a narrower focus than the average report. A report on the other hand is usually written either for an instructor, or for a committee. They usually have a little bit more knowledge about whatever subject they want you to write about, and expect more details and a greater amount of information, sometimes on a very deep level. This can get dry, but since the ultimate purpose of a report is the information, not selling it, accurate information is more important than holding the reader's interest... and in fact, the reader's interest may be held just because of the subject matter, because you rarely, if ever, write a report that hasn't been requested by someone specifically. Reports usually have a broader focus than articles, and their aim is to grasp the overall picture, rather than focusing in on a specific problem or interest area within the topic. Both should be accurate and cover the topic... you are just writing to different audiences who are going to be interested in different aspects of the issue, so the focus and the depth are going to change, based on the audience. Incidentally, if you are asking about the difference between "an" and "a" (since they were capitalized in the original question), the difference is that you use "an" before a vowel sound (Apple or hour), and "a" before a non-vowel sound (peach or pneumonia).