Technical aspect/explanation:

- EXIF is already there, seeing as that is written by camera. EXIF-info contains shutterspeed, aperture, time and date of the image and so on. Mainly it's not meant to be changed.
- IPTC is a pretty old standard it seems (1979 I read). It is used for adding tags and descriptions, like "horse, riding, sport, animal".
- XMP is newer, and based on XML. Seems to do about the same as IPTC, but I get the impression that it's the new open standard for metadata for images.

Read here for more on EXIF vs IPTC if that is still unclear.

I use Picasa to sort out images on disk, and upload them to flickr to store and show them online.

When tagging an image in Picasa, IPTC is used. flickr supports IPTC. You can try it yourself: Tag an image with something in Picasa, then use Uploadr for instance. And you can already see it being tagged with the tag or tags from Picasa.

Picasa and flickr both support IPTC and EXIF, but not XMP it seems.

What I have decided to do is just going for IPTC right now, and hoping and mailing about XMP-support. Getting flickr and Picasa to write tags in both IPTC and EXIF would be awesome, and I expect that either they, or someone externally will build a tool to do this when the time comes.

Note: There seem to be formats that XMP and IPTC don't support. Should think XMP supports more, like RAW.

If you have any questions about this, then Google Inc. has a great searchengine, or you could make your own thread linking back to this one.