The main difference between the internet and an intranet is that an intranet is a closed network. It isn’t publicly accessible to anyone on another network that the network its hosted - there are of course exceptions to this.
An intranet may be used in a couple of different scenarios, an example of a few are:
We could say in this case that the sole purpose of an intranet is to help employees with various tasks within a business or organisation. Intranet systems have evolved over the last decade, where previously they were use for the storage and distribution of files they have now evolved to carrying out a number of business functions.
Most corporate or larger businesses operate an intranet along with an internet site (Website). These are used in different ways, however both are useful for a business which is why they tend to both be implemented side by side. One provides public information and a means of digital marketing and the other keeps your employees and internal staff in the know.
Intranet’s are usually a little more secure due to the way they are networked - think about it, no public IP address which can be attacked, only “trusted” members of staff or internal people within your organisation can access this.
An intranet website uses the exact same technologies in most cases as a public facing (internet) based website. You should therefore expect an intranet application to be typically developed in HTML, CSS and a server side language such as PHP or ASP.