Most professors require scholarly journals (sometimes referred to as peer-reviewed or refereed journals) for research papers. Here are some general characteristics of scholarly journal articles and why they're useful for academic papers.
For a more complete list of characteristics and for help with distinguishing the differences between the three different types of journals (scholarly, trade, & popular), use this chart.
One of the easiest ways to find scholarly articles is to use one of the library's subscription databases. Most databases have search limiters that narrow your results to only information from scholarly sources. This search limiter option is almost always on the advanced search screen.
For example, if you are trying to find scholarly articles about music as a stress management tool, try using the library's Academic Search Ultimate database. Your search screen setup would look like this:
Five minute video explaining the peer-review process and why it's so important (from NCSU).