Wow, broad question. I'm not so sure I know how to answer it. Being a webmaster can include everything from developing, to designing, to promoting, to managing a website. Depending on the specific model, it can encompass elements of a community, of a business, of retail/commerce ... there's just so much.
Web design is actually designing the look and feel of sites. PHP and ASP are two backend server-side programming languages that are used to create dynamic web applications - this is the job of the web developer. I guess the only advice I could give is to decide what type of website you want to build, and research what you need to make it a reality.
I'd say there's a big distinction between the webmaster, the web developer, the content provider, the editor, and all other roles for most non-trivial websites.
As the site grows the webmaster will more and more have a supervisory role as a manager with business responsibility rather than being actively involved in the development process itself.