iOS and Xcode
What is Xcode?
Xcode is the development tool for creating iOS apps
iOS is the operating system that runs on Apple's iPhone and iPads. Xcode is the free tool that you can download from Apple for creating and developing iOS apps. There are other tools and applications for developing iOS apps that you could use, but this is recommended by Apple, and is the tool that they use themselves!
Visit the Apple's Xcode site to learn more.