A source may be defined as any kind of information from someone else that a writer draws upon to advance his or her own ideas. Types of sources include print sources, such as books, magazine and journal articles, as well as nonprint sources, such as computer software, movies, television programs, and interviews.
Sources are useful--indeed, sometimes crucial--when a writer needs to express ideas outside his or her realm of experience. Because good writing requires an attention to detail, sources often are needed to provide the substance that transforms boring, abstract writing into vivid, engaging writing: anecdotes, examples, illustrations, analogies, comparisons, as well as factual information such as statistics are all elements of good writing that can be derived from sources.
However, there are still certain questions a writer needs to address when using sources. One thing to consider is the intended audience of your piece of writing. What do they need to know about your subject matter? How much background information will be needed for your audience to understand your subject?
Also, you should consider your own knowledge of your subject. Even if your topic is something you know well, you may need additional sources to provide examples, additional information, or to lend authority to your own ideas and claims. For instance, you may know a lot about television programming, but some well-placed use of sources from television critics--specialists in their field--would add to your credibility as a writer and suggest that others who are experts share your opinions.