Picasa Web Albums 3, has many updates; improvements to keywording and tagging. You probably knew about Tag button next to the Search field, but did you also know that CTRL - K, and CTRL - T bring up the tagging dialog? While editing photos, this keyboard shortcut is very convenient to help you add tags on the fly.
The blue status bar at the bottom of the screen now shows any tags applied to your pictures -- this works on the single image, and the album level.
Tags are also a thumbnail caption option now! You can try it out in
View > Thumbnail Caption > Tags.
Multi-word tags are now supported. In Picasa 2, "happy birthday" would become "happy" and "birthday". Now it works like you want it to. Last, but certainly not least, bulk tagging is faster and there’s also an “auto-complete in the tags dialog, and in the Search bar.”
Source:→ Google Photo Blog