Precisely what primary versus secondary means can vary a bit depending on which field you're in, but a secondary source is always analyzing one or more primary sources. Contemporary descriptions or comments from during the period of creation or event by people who witnessed an event can also be considered primary - even if they wait for years to share it.

Secondary sources involve interpretation and analysis of the primary object or event. In general, the farther removed from the creation of an object or witnessing of an event someone gets, the more likely anything they create or write about it will be considered  'secondary'.

Tertiary sources tend to synthesize both primary and secondary information, and tend to be created even later, when what has occurred is better understood. Typical tertiary sources are encyclopedias, textbooks, handbooks, and other reference sources.

Examples:

In History, a diary written during the 18th century in rural New England would be primary; a scholarly research paper or monograph written in 2006 describing the kinds of topics appearing in rural 18th century New England diaries would be secondary. An entry in an encyclopedia about rural 18th century life that also mentions diaries would be tertiary.

In Business, the data from several focus groups held with people considered to be one company's new target market would be primary information...as would the market report submitted soon afterwards to the company leadership. However, a news report written months later documenting that industry's general expansion into a new market would be considered secondary information. If the way that company expanded was considered a classic example of how to do it effectively, and how it was done was mentioned in a handbook of business 'how to' strategies, that handbook would be a tertiary source.

In English Literature, a novel would be a primary source, and the paper written 50 years later about the author's themes in said novel would be a secondary information source. The book review written right after the author first published the book, however, would also be considered primary. But a later handbook about the literature movement that the author was considered a member of would be tertiary.

In Biology, a research study describing an unusual organelle observed via electron microscope in a particular type of bacterium would be primary research information...while a review article compiling and synthesizing all the new cell organelles discovered within the past decade, written five years after the research study's publication... would be an example of a secondary research study/source. A microbiology textbook that included that information a few years later would be tertiary.

In Art History, a work of art - and reviews, books by the artist, written material about it by contemporaries/friends, is the primary source. Later analysis is secondary, and established information about that work will eventually end up in handbooks, textbooks, and encyclopedias, tertiary sources.

In Anthropology, it can be a little nebulous...as essentially anything produced by a particular culture is be fair game to be considered a primary source (phsysical artifacts, structures, languages, customs, etc.), available to be studied, analyzed, and written up into a secondary information source. However, an anthropology textbook or handbook mentioning information now considered to be established and reliable would be tertiary.

  Don't get too distracted by the format, the form of the information source.  Whether it's primary, secondary, or tertiary really depends on the CONTEXT.

An object or written source could be primary for one type of research, but secondary for another!

EXAMPLE: A biography about a famous historical figure could be secondary if you were researching that person...but could be primary if you were studying how and why authors keep writing biographies about that famous person!