Building web-applications is hard! Even the simplest hello world application will take you hours to set up and you have to know a lot about how many different pieces of technologies work together. Here is a quick inventory from the browser to the back end of what a typical web-application developer needs to know about:

• HTML,
• CSS,
• JavaScript,
• cookies,
• HTTP,
• URL encodings,
• Servlets,
• web-framework (i.e. web-work),
• authentication,
• threading,
• SQL,
• ObjectRelational mappers,
• DB Schema,
• RDMS,
• DB indexing.

If you know all of that, than you have a good chance that after a week of work, you can have a hello world application written which will allow the user to authenticate, and persist some data.

Source: Misko Hevery