Primary sources are the historical documents used by historians as evidence. Examples of primary sources include diaries, personal journals, government records, court records, property records, newspaper articles, military reports, military rosters, and many other things.
In contrast, a secondary source is the typical history book which may discuss a person, event or other historical topic. A good secondary source uses primary sources as evidence.
The key to determining whether an item may be considered to be a primary source is to ask how soon after the event was the information recorded. This can be a problem with an autobiography, memoir, reminiscence, etc. if the author is working several years with only the memory of what happened. Your history professor will disallow most or all of these as primary sources.
Who: Who is the author or sponsor of the website? Is that person or organization named? Is any supporting documentation available?
What: What is the mission or purpose of the website? Is it clearly articulated? What kinds of materials are on the website? Are they properly cited and acknowledged? What is the document format on the web?
Where: Where is the site located? Is there a physical address with phone number and email address for a contact person? Does the site have a .edu, .org, or .com address?
Why: Why does the site exist? Does it have a point of view or opinion? Is it pedagogical or polemic? Does it want something from you?