They can include original manuscripts, contemporary records, reports, photographs, documents used by an author in writing a book, patents, maps, statistical tables, diaries, letters, original artworks, journal articles reporting original research, etc. Primary sources are particularly useful in that they contain details that are often ignored or left out by the people who write the next sort of resource. Secondary resources occur when primary sources are compiled, modified or rearranged to produce material for a specified purpose or audience. Examples include biographies, reviews, histories, collections of abstracts, bibliographies, journal indexes, commentaries, analyses, criticisms of literature or art. Consulting secondary sources can save you lots of time, and you get the benefit of the author's knowledge and expertise.
Examples are encyclopedias, bibliographies, handbooks, guides, almanacs, and factbooks.
The distinction between secondary and tertiary sources is often hard to make. Perhaps the important point is that tertiary sources are usually another step removed from the original information. That doesn't mean that they are necessarily less accurate, but there is always the danger that each step away from the primary source material will lead to further errors of interpretation or fact. Sometimes your assignment will require you to work from the primary sources. At other times, you will find it much more useful to use secondary or tertiary sources.