Tags are like keywords. By applying single or multiple-word tags in Picasa, you can quickly search and locate your photos. To add a tag to a photo, please follow these steps:
Select the photos that you'd like to tag. You can only tag photos in one folder or album at a time.
Click the Tags button (a piece of paper icon located in the lower right corner of Picasa). You can also use the keyboard shortcut Ctrl-T (Windows) or Command-T (Mac). Existing tags and tag counts for selected photos will be listed in the 'Tags' tab.
There are two ways to add tags:
Type the tag manually: Use the text box to enter the tag and click the 'Add tag' button.
Use your 'Quick Tags': By presetting tags that you frequently use, you can add them with the click of a button. Manage your 'Quick Tags' by clicking the 'Configure Quick Tags' icon in the 'Tags' tab.
To remove a tag from all selected photos, just hover over the relevant tag in the Tags panel and click the 'x' icon that appears.
Once your photos are tagged, you can use the search box in the upper right-hand corner of Picasa to quickly find pictures that you've tagged.
If you'd like to view your tags underneath their respective photos in the Picasa library, adjust your thumbnail captions setting.