Keywords you add to pictures using Picasa are actually stored in the pictures themselves, so they are preserved if you copy the pictures elsewhere.

These are the same as the tags that Vista uses, except that Picasa stores each word as a separate “tag” and doesn’t allow for capital letters – loading Picasa tagged images in Vista Photo Gallery looks rubbish. In addition, Picasa doesn’t let you view groups of keywords by selecting from a list, or easily add a previously used keyword to a group of photos. I may think about a Flickr Pro account at some point.