Journals, or periodicals, are published continually on a scheduled basis, and can come in physical or electronic formats. These publications contain articles that are usually written by experts within a field. Journal articles are usually the best way to learn the most recent information on topics within a profession or field of study.
Books usually provide more comprehensive information on a subject, even if it doesn't contain the most recent information. Textbooks are a good example, they provide foundation level information that can be built on with newer information. Like journals, books can also come in electronic formats, or ebooks.
Databases are electronic warehouses of information, usually with a searchable interface. Databases usually contain articles from journals or ebooks, but some databases (i.e. UpToDate or Micromedex) just contain a specific set of topics that are constantly being updated as new information is being discovered.
Websites found via search engines can provide a basic understanding of a topic when starting research, but should not be exclusively relied upon for quality research. Remember to use a set of criteria, such as the SMART evaluation, to make sure a website is providing reliable information.
A journal that publishes articles written by experts. These experts usually have either a degree or years of professional experience in a particular field. Depending on how the journal checks an article for quality before publication, the reliability of the journal may vary, despite being written by "experts".
A journal with articles written by experts and reviewed extensively by a group of other experts. Usually the author doesn't know who is reviewing their article and the peer review group doesn't know who the author is. This anonymity is called "double-blind" peer review and is used to prevent undue bias from affecting the review process.
A source that provides a direct perspective on a research topic. For medical and scientific fields, this includes journal articles with original research conducted by the author. For literary and historical fields, this includes original manuscripts or first-hand accounts of a story.
A source that provides perspective that is one-step removed, or filtered through someone else's perspective, on a topic. For medical and scientific fields, this can be a literature review of different studies conducted on a topic. For literary and historical fields, this could be an essay someone else has written about a topic.
A source that combines information from both primary and secondary sources on a topic and synthesizes it into a brief summary or description. These sources represent the current general understanding of a topic. Examples include dictionaries, encyclopedias, and fact sheets.