From Apple's website: "Apple Watch works with iPhone 5 or later running the latest version of iOS, which introduces the Apple Watch app. So make sure everything is up to date"
Note: While it does say "Requires iPhone 5 or later." in the footnotes at the very bottom of the http://www.apple.com/watch/technology/ mentioned by Matthieu Riegler, nonetheless it's important to note that both the iPhone 5 and later must have at least iOS 8.2 or later. So an iPhone 5 still running iOS 7.x will not work with the Apple Watch.
You can still use it. The only thing the phone allows it to do is call, receive texts, check distance you've gone (GPS) or some other key features like camera. Everything else is still accessible to to it being able to connect to wifi.