Finding aids are created by archivists to describe the materials in an archival collection. They will provide context for the primary resources, background and/or biographical information, a summary of the items, and they will also offer organization and location information. When accessing a digital collection, read through the finding aid to understand what is included in the collection. Bonus: the finding aid may offer information you can cite in your work, too.
The physical format of the primary resource. Sometimes the format is as important as the information it provides. Especially when you are viewing digitized collections, it can be helpful to understand the materiality of the primary sources you are viewing. True story: A pair of (ahem) unwashed underwear was found in Andy Warhol's private archives many years after his death. Their format and inclusion in the collection added to Warhol's life story!
Materials that serve as evidence to document an event, time period, people, idea, or work. They can occur in many formats: books, manuscripts, diaries, photographs, ephemera, films, recordings, clothing, personal items, emails, Instagram posts.
Although archivists and librarians usually strive to maintain a low level of bias, archives and libraries are not neutral! Collections in their holdings are curated by people who make decisions on what to include and what not to include. For this reason, there may be gaps or missing pieces and some voices may be suppressed. Sometimes these choices are made because items can't be located, sometimes it is a space issue, and sometimes it's because the voices of others are just not valued enough for inclusion.
A surrogate is a copy. When an item is copied to preserve the original or is copied to be placed on the web, what you are seeing is a surrogate--a copy--of the original. In most cases, the item is copied or photographed identically, but it is important to remember that what you are viewing in a digital collection is a copy of the primary resource and not the original item in its true form.