Backing up Gmail content can be done by using email client like Thunderbird or Microsoft Outlook, but not everybody is willing to use them just for the sake of backing up mails. Lifehacker pointed out how to backup Gmail mails with the use of fetchmail but I think some people will not feel comfortable to engage with this application because it involves scripting, Unix commands and configuration file changes; well, unless you are interested in learning something new.
Gmail Backup is another alternative solution that will backup all Gmail mails and mail folders with all labels, from fields and dates. Before initiating the backup, make sure to enable IMAP in the Gmail settings. You can also specify a time interval so that emails that come in during a certain period of time will be only backed up and not all of them.
One interesting feature in Gmail Backup is it stores the backup in Microsoft’s eml format which allows user to easily import the messages into other mail clients.