With the file extension PSA so-called Adobe Photoshop album files are marked. Photoshop Album is photo management software with rudimentary image editing features. 2004 appeared the last version. In 2009, the free (but registered) Starter Edition was released, but their support was discontinued, which is why the registration process no longer works.
A PSA contains, among other things, the tags and captions assigned in the management software. Psa files can be imported from Adobe Photoshop Elements (from the Organizer), so you can preserve the structure, tags, etc. when importing image collections. The PSA file is usually located under Documents and Settings in the hidden ApplicationAdobe Photoshop Album Catalog folder.