You can send through this page an bug report.
The report should include enough detail, so that the developers can repeat the bug scenario.
You also need to define a severity and priority to the bug. Severity means how bad the bug is in your opinion. For instance, a blocker could mean that the service makes your Web-browser to crash or loses some of your data and you will not use the service until the bug is fixed. On the other side trivial can mean that you have found some typing error.
Priority means how fast you see that the bug should be corrected. Usually for a blocker, critical or major bug the priority is high. But, it can be that you see a typo also as an bug which should be urgently corrected.
Add to the form your name and e-mail address if you want to be contacted concerning the bug.