Safari, like all modern web browsers, caches data locally not only to speed up access to frequently visited sites, but to let data persist for web apps - including sites like gmail.com - so they can perform more like native apps. However, not only can the stored data build up over time, it can contain private information, or even reveal the names of websites you visit, that you might not want found on your iPhone, iPod touch, or iPad (hey, we don't judge!). Luckily, iOS 7 lets you quickly and easily delete this information - once you know where to look!
That's it. All Safari data should be deleted now. If you're more worried about site tracking than anything, make sure you block website tracking as well!
Note: Clearing website data will also clear any saved logins or passwords you have stored inside Safari itself. If you've got it saved in iCloud Keychain however, that's stored separate and will remain.