I'd be willing to bet that a good number of beginner genealogist and probably a surprising number of more experienced genealogists don't have a good grasp of the different types of sources of information. I know I never really thought of it before and prior to really thinking about it, I'm pretty sure I had a couple of these types confused. There are six categories you can put sources of information into. These are three sets of opposites. Any given source can be classified as one of each of these pairs so each source is actually in three of these categories, all at once.
The first pair of categories I'm sure most genealogists have heard of. These are Primary and Secondary. A primary source is one where the information that it contains was documented first-hand by a person with direct knowledge of the event. A birth certificate is a good example of this. A doctor that was present at the birth of a child is the person that creates the birth certificate. He has direct knowledge of the date and time and the name of the mother. Because the document is created near the time of the event, this is considered a primary source. A death certificate is also a primary source for the date, time and place of a person's death, created by a doctor with direct knowledge of the death. A death certificate also contains the birth date of the individual, but in regards to the birth date, this is a secondary source. The person with direct knowledge of a person's death is unlikely to have direct knowledge of their birth also, except in the event of a stillbirth or a child that dies within hours or days of their birth. Also keep in mind that scanned and photocopied documents, as long as they are scans of the original document, are considered primary sources.
A secondary source is a piece of evidence that states a given fact but, since it was not created at the time of the event or by a person with direct knowledge of the event, it is not considered a primary source.
The next pair of evidence types can be confused with primary and secondary. These are direct and indirect sources. While a birth date on a death certificate or in an obituary is not a primary source, it is still a direct source. The death certificate or obituary states that the birth date is a certain date is directly stating this fact, so therefore it is a direct source. A death certificate or obituary that does not give a birth date but contains someone's age would be considered an indirect source.
A direct source of information states the fact you are looking at directly, even if the person creating the document doesn't have direct knowledge of that event. An indirect source is something where it contains information but does not state it outright. It requires you to do some thinking to come to the conclusion.
The last pair of evidence types are original and derivative. An original piece of evidence is the original document containing the information, whether that is a passenger list or a census or about any other type of evidence. A derivative source is one that is derived from an original piece of evidence. A passenger list index or a census index would be considered a derivative piece of evidence. Someone that had access to the original document created the derivative document, but you're not looking at it. You are depending on that person's interpretation of what he or she was looking at.
Like I stated at the beginning any piece of evidence can be categorized into three of the above listed categories, one from each pair. A given document might be an original, primary, direct piece of evidence, such as a birth certificate, or it might be an original, secondary, direct source, such as a birth date on a death certificate, or it might be a secondary, indirect, derivative source, such as an indexed list of students, listing their age.
While you don't need to categorize every piece of evidence you use while working on your genealogy, it's good to be able to look at any given document and determine how you would categorize it so you can determine which piece of evidence is more likely to be accurate when you have two conflicting pieces of evidence.