A source which provides first-hand evidence about an event, object or person is called a primary source. You should always use the primary source when citing, as you are expected to have read and evaluated your references.
In Sciences, primary sources are often the results from empirical studies, which are typically found in research articles or conference papers. The primary source can also be the raw data behind the empirical study in the form of surveys, fieldwork and statistical data. Other examples of primary sources are government documents, reports from public authorities, historical and legal documents, eyewitness accounts, correspondence, blogs, opinion-papers, interviews, speeches, audio and video recordings, and artworks. However, some of these sources are primary sources only when citing opinions and statements, and scientific publications alone can be used as primary sources for scientific facts.
Secondary sources do not contain any first-hand evidence, instead they summarize, describe, discuss, analyze, evaluate, and in other ways process, primary sources. To cite a secondary source is generally to be avoided, since you cannot be sure that the author(s) of the secondary source gave a true representation of the primary source you want to site. If it is impossible to get access to the primary source, i.e. if it is rare or if there is a language barrier, you may in some cases use a secondary source.
The most common secondary sources that you will encounter in Science are lectures, review articles, compilation theses and reference materials such as textbooks, special topic books, dictionaries and encyclopedias. Other examples of secondary sources are articles in newspapers and popular science magazines, popular science books and book reviews. It is also important to remember that websites, blogs, opinion-papers, interviews and speeches by researchers are secondary sources when citing their research and not their opinions.