Archives & Manuscripts is a database of collection guides describing archival and manuscript collections at Bryn Mawr College, Haverford College, and Swarthmore College. It is located at http://archives.tricolib.brynmawr.edu/.
If you are familiar with searching Tripod in order to find books, Archives & Manuscripts is a similar concept to help you find collections of historical materials. It is specially designed with special features that make it easier to browse archival descriptions.
If you are searching for digitized materials, like scanned photos and manuscripts, you will find those at http://triptych.brynmawr.edu.
Archives & Manuscripts uses software called ArchivesSpace, which is utilized by many archives across the US and indeed the world. Our colleagues at other institutions have created wonderful resources that may help you understand how best to use the tool.
A finding aid serves as both the table of contents and index of a manuscript collection. It will help you determine whether the collecion has material of interest to you, and, if so, where that material is located.