Primary sources were written or created in the historical period under investigation or soon thereafter. Official or private documents, such as letters and diaries, contemporary newspapers, and magazines are all regarded as primary sources. Carvings, paintings, illustrations, sculpture, clothing, tools, architecture can be interpreted for their meaning and revelations about human civilization in a given epoch.
Primary sources are ALWAYS the foundation for solid historical research, as every academic secondary source is firmly grounded to primary sources in someway (otherwise in effect the author is simply making it up as he/she goes along). This is not to say that primary sources should be accepted without hesitation, as no one who has ever created a primary source has done so as a perfectly neutral actor or with a perfect comprehension of their own time. Instead primary sources much be read critically and skeptically with the relevant constraints in mind such as the authors agenda, the lack of information known to the author etc.