When you participate in a thread on the forums, by default, you'll receive email notifications when someone else replies to the same thread.  If you'd like to not receive an email notification, you have two choices:

Turn off Email Notifications for the one Thread

Click the "Stop Following" link to the right of the first post in the thread:

Turn off All Email Notifications from this Website

Click "Edit" (top right) to edit your profile, click the "Site Options" tab...

Then, under "Email Configuration", set "Send Notifications" to "No".