A primary source is a first-hand or contemporary account of an event or topic.
Primary sources are the most direct evidence of a time or event because they were created by people or things that were there at the time or event. These sources offer original thought and have not been modified by interpretation. Primary sources are original materials, regardless of format.
Primary sources may be transformed from their original format into a newer one, such as when materials are published or digitized, but the contents are still primary. There are many primary sources available online today, but many more are still available in their original format, in archives, museums, libraries, historical sites, and elsewhere.
Secondary sources usually use primary sources and offer interpretation, analysis, or commentary. These resources often present primary source information with the addition of hindsight or historical perspective. Common examples include criticisms, histories, and magazine, journal, or newspaper articles written after the fact. Some secondary sources may also be considered primary or tertiary sources - the definition of this term is not set in stone.
Tertiary sources are further developments of secondary sources, often summaries of information found in primary and secondary sources and collecting many sources together. Some examples of tertiary sources are encyclopedias and textbooks. Again, this term is not set in stone - some sources may be both secondary and tertiary.