Brace yourselves, iOS 8 is coming soon.
But seriously. Before upgrading to iOS 8, make sure you have a complete backup of your device if something goes wrong.
KB on backing up your device with iTunes or iCloud: http://support.apple.com/kb/HT1766
iOS 8 will work on the following devices: iPhone 4s, iPhone 5, iPhone 5s, iPhone 5c, iPod touch (5th gen), iPad 2, iPad with retina display, iPad Air, iPad mini, iPad mini with retina display.
As always with a major upgrade like this, the servers will more than likely be hammered when iOS 8 first appears. Be patient and keep trying throughout the day, or wait until the weekend when you have more time to dedicate to backing your device(s) up before upgrading.
A lot of apps will also be updated either today or in the next few days to take advantage of the new features of iOS 8.