1. An "intranet" is a website for internal purposes only, that usually covers issues relating to the company itself. It can contain recent events for the comapny, newsletters, personnel matters, events .... basically anything that is curernt and relevent to the company.
2. There really isn't anything different between an internet site and an intranet site, as far as the coding is concerned. Intranets are for internal use only - no-one outside of the company (the general public) will not have access to a company's intranet.
3. Well, it is good experience for web design and web development. If it is for a project, ensure that you plan the development and your code is well documented - don't just start coding away, simply because your tutor won't be impressed with your project management skills !!