Truecaller is one of the few apps that tell you the name of the person calling you and the number of that person is not saved in your phone. There are also few more apps that claim to do this but those apps are not reliable. Truecaller is used by billions of people all over the world and it is very reliable app, the user interface is really good there will be no problem to understand this app it is not complicated also.
You can just download the app and register and thats it whenever you recieve a call it will show you the name of the person in 1-2 seconds depending upon your internet connection and yes you need internet connection to work it and once it identifies the number then it can show the name even if you are not connected to internet. SO very good app must use as now a days spam calls are big issue.