As Erin said above, they'll be used as the 'backbone' behind some new features she's working on.

However, in 'standalone' mode as currently implemented while the feature's being tested, buttons are automatically awarded for interaction with the site (logging in, posting stories / blogs, commenting on stories / blogs, kudo-ing a story etc.), while the Give Buttons link allows you to donate some of the buttons you've earned (but no more - you can't have negative buttons!) to someone else.

There have been two previous threads on the subject:
http://bigclosetr.us/topshelf/forum/35265/what-is-button
http://bigclosetr.us/topshelf/blog/35206/okso-ive-got-askbut...

...which also have lists of how many buttons you currently get for certain actions. Note, however, that as the feature is in development, any / all aspects of their current operation is subject to change without notice.

I'm sure that once Erin has fully implemented whatever she intends buttons to be used for, she'll post a blog informing us - and probably post-date it so allowing it to remain visible for a few days (or up to a week), thus giving most regulars the opportunity to read it before it disappears.