Weather is the built-in iPhone app that lets you monitor the temperature, precipitation, and other climate conditions in your local city and cities around the world. The original version shipped with first iPhone and looked a lot like the old Mac dashboard widget. Powered by Yahoo! weather data, but for the performance problems of mobile web apps at the time, a widget is what it could have ended up being on the iPhone as well. While Weather has had a few facelifts over the years, including a major one to match iOS 7's new design language, some nifty animations aside, it's pretty much now what it's always been — just enough. That's probably why the market for third-party weather apps on the iPhone is so strong.
Apple hasn't shipped a version of Weather for the iPad. Rumor has it they didn't think it would suit the bigger screen. The depth and quality of App Store iPad weather apps has long since proven that false.