Archives are a special kind of library which holds a wide variety of materials, both published and unpublished. Often archives focus on a specific subject area, or collect materials from a specific geographical region or a particular instruction. For example, Seattle University's Special Collections collects, preserves and archives materials related to the university and provides access to the collection. Some examples of materials found in archives and special collection are manuscripts, letters, photographs, moving images and sound materials, artwork, books, diaries, artifacts, and the digital equivalents of all of these things. Materials in an archives are often one-of-a-kind which means preserving and providing access to these materials is an archive's primary function.
This page contains links to local and specialized background archives and resources repositories that you will use in your research. Keep in mind that all these databases and archives will have different interfaces and ways of organizing their (sometimes very different!) information and materials. Be sure to explore the website before you start intensive searching. Familiarize yourself with the interface and read the "about" section of the website to get a sense of what sources you may find and how to search. You may also encounter instances where there is a listing for an item or resource online, but the item has not been digitized, such as a magazine article or a peice of artwork. This means that the item exists only physically and you will need to schedule a visit to the archive or submit a request to have scans of an item sent to you. Different archives may have different ways of providing access to their materials, but web databases usually have an "about" section that tell you how to request access.
A finding aid describes what is in a collection and how the collection is organized. A finding aid also gives context for how, why, and by whom the documents in the collection were created. Finding aids can be helpful tools for researchers in determining whether or not a set of records is relevant to their topic, and can also help them navigate the collection to find the records most likely to be useful for their research. Contents of the collection will be listed at the folder level, as archival documents are generally not cataloged item by item.
For further reading as you continue your research, see this comprehensive guide to understanding and using a wide variety of archives, by the Society of American Archivists.