The SharePoint Framework (SPFx) is Microsoft’s latest addition to the SharePoint development family. It is custom web-part and page extensions programming model aimed at replacing the over-used (and potentially dangerous) Script Editor Web Part (SEWP) and Content Editor Web Part (CEWP) in SharePoint Online (SPO) and recent versions of on-premise SharePoint too.
TypeScript is a relatively new language that was created by Microsoft. It is an open source language loosely based on JavaScript syntax but extended to provide strong-typing declarations. An immediate advantage of using explicit data-types (like you would in other formal programming languages such as C/C++, C#, VB.NET, etc.) is that you get design-time data type checking when the ‘code’ is compiled into JavaScript.