Unlike other popular webmail services like Windows Live Hotmail or Yahoo Mail, Gmail doesn't support email folders in the traditional sense: this means that unless you regularly archive your email messages (a topic we'll cover in depth in another tutorial), your Gmail inbox quickly starts overflowing with old emails you no longer need. Like other email providers though, Gmail offers a "Trash" folder where you can put messages you no longer want to keep. This tutorial explains how to delete emails in Gmail.
Follow these steps to delete an email or an entire conversation's worth of messages in your Gmail account: note that this tutorial explain how to do this inside a browser, not for the special case where you are checking your Gmail account from an email program (in which case you would delete your messages as is done inside that particular email client).
Caution: emails you delete in your Gmail account will be moved to the Trash, and remain there for about 30 days. After that point, deleted messages will be completely erased, and become unrecoverable. To restore delete emails before that 30-days period is up, please read the following tutorial: "Recover deleted emails in Gmail: restore / undelete messages from the Trash".
Tip: you can also delete an email message when you are inside it, reading its content. To do so, click on the "Reply" dropdown menu, and choose "Delete this message".
If you don't like deleting emails while reading them, Gmail also lets you delete multiple emails at the same time.