Photo Gallery specifically uses the IPTC "Keywords" meta tag.
Google Picassa also uses this. IPTC tags are an extension to the data that your digital camera uses to store meta information inside the photo file (like date, exposure, etc.).
So if you are using either of these tools or any of the other tools that make use of EXIF and IPTC (such as the excellent GeoSetter), the data will automatically be backed up if you back up the file.
However, a word of warning. Watch out for image editing utilities that do not understand this data. This used to be a common problem on the iPhone. They may loose the meta-data all together.
For reference, there is an alternative method for storing image meta data called "XMP", originally created by Adobe. With XMP, I believe that the data can either exist within the image file or be held in a matching external file with a .xmp extension.
1Make a copy of a file before it's tagged, tag it (or let it be tagged), then compare that version with the original. – martineau – 2013-02-24T17:00:25.760