This is an advanced topic, and is not necessary to understand to use PhotoMesa. But we explain here how PhotoMesa stores its information so you can feel comfortable trusting PhotoMesa to access your valuable photos.
PhotoMesa respects your photos - it doesn't move them into a special "database" of imported photos. Rather, it lets you work with your photos inside of our outside of PhotoMesa as you choose - in your original folder structure.
PhotoMesa stores the annotations you create in two places - in the "header" of the photo itself, and in a single plain-text XML file stored (by default) in My Documents\PhotoMesaLibrary.photomesadb. The XML file is really just a backup - so if it gets destroyed, all the annotations will get regenerated from the photos themselves. In this way, if you share a photo that you have annotated with someone else, and they load the photo into their own copy of PhotoMesa, it will immediately show up with the annotations you made.
The annotations are stored in the photo header using a standard called IPTC. Since some other photo management programs may also use this standard, you may be able to see some of the annotations made within PhotoMesa inside of other programs.
PhotoMesa also creates a number of small "thumbnail" files that are very small versions of your larger photos. These are used to speed up PhotoMesa since it is faster to load and display the small files than the full-sized ones. The thumbnails get regenerated as needed, so it is always safe to completely delete those thumbnails. The thumbnails are stored in a standard Microsoft Access database in the file photomesa-thumbs.mdb in the directory that PhotoMesa is installed (usually \Program Files\Windsor Interfaces\PhotoMesa).