You can optionally add three kinds of identifiers to selected files to make them easier for you to find in searches: captions, star ratings, and keywords.
Add captions. Every image can have a captiontext that identifies the photo's subject, when/where it was taken, or something funny, for example.
Mark photos with a star. Chapter 3) allows you to display only starred pictures.
To mark the selected photo(s) in the Picture Tray, click the Star icon. (Note that the Star icon is present both when working in the Library or when editing a picture.) To remove the star from selected images, click the Star icon again.
Assign keywords. Another way to make it easier to find pictures with a Picasa search is to assign keywords to them. A keyword is a single word that describes an aspect of the picture, such as its subject. You can assign one or multiple keywords to an image.
Assign or remove an image's keywords in this dialog box.