AND combines search terms so that each search result contains all of the terms. For example, travel and Europe finds articles that contain both travel and Europe.

OR combines search terms so that each search result contains at least one of the terms. For example, college or university finds results that contain either college or university.

NOT excludes terms so that each search result does not contain any of the terms that follow it. For example, television not cable finds results that contain television but not cable.

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Primary sources are documents, images or artifacts that provide firsthand testimony or direct evidence concerning an historical topic under research investigation. Primary sources are original documents created or experienced contemporaneously with the event being researched. Primary sources enable researchers to get as close as possible to what actually happened during an historical event or time period. A secondary source is a work that interprets or analyzes an historical event or period after the event has occurred and, generally speaking, with the use of primary sources. The same document, or other piece of evidence, may be a primary source in one investigation and secondary in another. The search for primary sources does not, therefore, automatically include or exclude any format of research materials or type of records, documents, or publications.

As an example, a diary from an immigrant from Vietnam to the United States documenting her travel experiences from Vietnam to Orange County would be considered a primary source for research on Vietnamese immigration to Orange County. However, a book written by a professor that analyzes the various writings of Vietnam immigrants and interprets the experience of those immigrants is a secondary source for this research.

Source:UCI Library website
https://www.lib.uci.edu/what-are-primary-sources