Many archives preserve and make accessible diaries, letters, photographs and other primary source documents. Archives sometimes contain extensive and "hard-to-find" secondary source collections. Examples might include specialized collections of research books and articles on specific topics. A digital archive is similar in purpose to a physical archive, but the historical documents and objects have been digitized and made available online. Digital archives are usually created with a goal of preserving historical objects and making them broadly available to researchers.
Important to Note: Just because some collections in an archive have been digitized doesn't mean everything has! Pay attention to how things are organized and tagged as your search.
There are several reasons to organize collections thematically. "The thematic framework allows for coherent aggregation of content. All the materials included assist in research and study on the theme. This coherence is generally anchored by a core set of primary sources. The capabilities of networked, digital technology make it possible to bring together extensive corpuses of primary materials and to combine those with any number of related works. Thus the content is heterogeneous in the mix of primary, secondary, and tertiary materials provided, which might include manuscripts, letters, critical essays, reviews, biographies, bibliographies, etc., but the materials also tend to be multimedia. The digital environment provides the means to integrate many different kinds of objects into a collection." (Carole Palmer, Thematic Research Collections, A Companion to the Digital Humanities)
Thematic collections help provide research support, provide scholarly contributions, contextualize works, and make an interdisciplinary platforms. The collection of varying materials helps create a broader mass of research functionality and digital archives can help provide a nuanced research perspective for students to utilize.
Just because an item is in an archive, even a digital one, copyright still applies. You should always cite the archive in your research paper or other research project.
You should also ask before republishing something found in an archive. You may need to obtain permission to reprint or republish the material.
Archival citations vary, but good examples of citing archives can be important to see.