What They Are
Use Them When You Need...
Works of an "official" nature, serving as an authorized record of an event, decision, agreement, transaction, or other activity.
While any piece of writing is technically a document, in the library world it's reserved for writings intended to serve as evidence of something. Most documents are produced by governments, but corporations (including non-profits) and individuals also produce documents.
Documents were one of the first content types to move online. Today most new documents are online and many (but not all!) older ones have been scanned in.
However it's important to distinguish between "published" documents, intended to be widely distributed, and "unpublished" documents, the internal day-to-day paperwork of government and business. The latter group is rarely available online. Recent unpublished materials typically have only one or two copies residing in file cabinets or servers, and are often considered private or proprietary. Eventually such documents are either deleted/thrown away or deposited in an archive of some sort, at which point they're likely to be the only copy in the world.
Documents are almost never considered scholarly sources. Technical reports may be considered an exception, though they're rarely peer-reviewed.
By definition, documents are primary sources for studying the activities of the organizations that produce them. However, they can also be secondary sources on other topics—for example, government agencies often produce informational reports summarizing data collected elsewhere.
Governments produce voluminous documentation about many topics. When using government documents it's important to note which government agency produced them, especially in the US where government functions are distributed among multiple levels of government: federal, state, county, and city.
The judicial system produces a great quantity of documents: judicial opinions, court transcripts, and all of the documents filed by parties to a particular case.
Most documents produced by private businesses are private, internal documents which are rarely published. But a few categories, such as annual reports and SEC financial reports, are publicly available.
As the name implies, most technical reports are about research in the physical sciences or engineering. However, there are also technical reports produced in the life and social sciences,
Technical reports are documents that describe the results or status of a specific research project, most commonly produced for the sponsors of the project. As such they're technically internal documents that don't always see general publication. However, some research sponsors, most notably the US federal government, allow (or require) public distribution of their technical reports.
Like conference papers, some technical reports are eventually transformed into academic journal articles, but they may also be released after a journal article to provide supplementary data that didn't fit within the article. Also like conference papers, technical reports can be hard to find, especially older reports which may only be available in microfiche. Ask for help from a librarian!