Hi
I think it's quite simple:
Design - the act of sytematically planning something, normally in words and images, to achieve something. Could be a web site, boat, building, car, ect.
Development - the act of developing - bringing something, like a design, into being.
From a web point of view, your client comes to you and says 'I want this, that and the other'. Your initial steps are to sketch out a design, which includes ideas, technical bits, colours, accessibility, and so on. Your next step is to bring that lot together and make it happen - develop it. You may choose a template if that meets your design criteria, but that is still in the development domain.
So, if you do these things, you are both a designer and developer. That means you get double the pay for both jobs, right?
Regards
Paul