A source is any piece of information you use when you write that is not from your own head/idea.
Failure to acknowledge/attribute/document a source = plagiarism
Professors will ask use to use various styles to cite sources, the most common being MLA and APA.
The library has a guide to help you cite your sources.
Citing your sources remains an incredibly important and key portion of the research process. The need to cite every source may be different from the process you used in your home country. It is your responsibility to know and use the process for citing sources that is mandatory at U.S. education institutions.