Scholarly sources are written by experts in the field of study using technical or academic language. Very often they are peer-reviewed before publication, and they will always have a full bibliography. They are usually considered to be credible sources.
Popular sources are written for a more general audience. They are most likely to be written by journalists and may not have a bibliography. Their language is generally less formal, and they might be written from a biased point of view. This means that these types of sources are not always reliable, but can still be used and cited if appropriate to your research.