Images can have various data associated with it. This data can reside in different places. Depending on where the data resides, Shozam can read it and display it. Currently Shozam reads EXIF image data.

Your images may have data associated with them. This was either done automatically (e.g. a digital camera tagging information in the images it takes), or manually (e.g. using some image organization software to group and tag or label images). Please note that this data is either stored in the image file itself (using a standard such as EXIF or IPTC), or outside the image in a database.

Databases are generally used by image organization software, providing those programs a more flexible way of storing all sorts of information. Unfortunately, databases have major setbacks - lack interoperability (no standard way of doing this) and ease of maintenance (data and images are stored in different locations).

EXIF standard tags include: camera make/model, aperture, date, longitude/latitude, resolution, title, description, software, exposure, etc. For a comprehensive list of tags Shozam can read, go to the image tab in Step 5 of Shozam

IPTC standard tags include: caption, keywords, headline, city, copyright, etc.

Proprietary tags typically include: keywords, subject, caption, people, places, etc. Note that each image organization program is free to create their own tags how they wish. This flexibility unfortunately makes those tags proprietary and incompatible with other programs. Also, the fact that those tags are not stored in the image but in a database further complicates interoperability and portability of this data.

As creating and maintaining this organization data for images become increasingly important to users, I expect more image management programs will move away from storing organizational info in databases and move them into the standard IPTC metatag and part of the image file. Another open standard that is rapidly gaining ground is XMP - Adobe's Extensible Metadata Platform. Adhering to standards and storing the metadata in the image file itself will ensure interoperability between programs and much easier maintenance since the data will be part of the image file rather than stored in another location.

Additional resources:
http://graphicssoft.about.com/od/glossary/f/metadata.htm
http://graphicssoft.about.com/od/exifsoftware/EXIF_IPTC_XMP_Software_Metadata_Readers_Editors_Extraction_Tools.htm
http://www.adobe.com/products/xmp/index.html
http://www.controlledvocabulary.com/imagedatabases/iptc_naa.html
http://www.controlledvocabulary.com/imagedatabases/iptc_naa.html#IPTCchart