Google Buzz is a social networking tool built into Gmail. It allows you to stream feeds from other sources like Twitter and Google Reader and make public or private comments on them. You can also make public or private "Buzz" posts which can be commented on by anyone who can view them.
Public Google Buzz posts show up in your Google Profile page.
To use Google Buzz, log into Gmail and click on the Google Buzz link on the left side. You can click on your inbox to resume using Gmail as usual.