I don't use qlikview I use qliksense, but I think they are similar. So mashup is an embedded web page in a web application. In Qliksense, it is used to show visualizations in your web application. API are application programming interface, this teaches you different methods and functions found in a framework, so qliksense this teaches you how to use different features of qliksense. To creat mashup in qliksense you can look at these links that might help with
