Napa - Office 365 Development Tools is a great way to get started building apps for Office or SharePoint directly out of a browser window. There is no need to install any other tools such as Visual Studio.
All you need is an Office 365 account and a supported browser. To get started, sign up for an Office 365 Developer Site. Then, install Napa - Office 365 Development Tools on your Developer site and you are ready to create apps for Office documents, mail items or SharePoint.