Oh I can't believe such a question is asked on this site avout internet even at this year of 2017. I feel it as just like asking a question like what is a book or something else like that. But I know every question is a question that have or need an answer.
Well, An internet is a network that connection websites, servers, computers and people all over the world. How you asked this question on this site? There is the answer. That is an example of usage and benefit of internet. And about types what do you mean. I think you are asking about www, the world wide web thing. Any private network or public network that are closed to a region or institution can't be called an internet unless it is connected to the world wide web. Instead they are termed as Intranet.
To use an internet you need an internet connection from any ISP in your area and a modem to connect it to you (usually provided by ISP's you are connecting to) and a device like PC, mobilephone, tablet etc to access the internet. You need to configure your device to connect and need an Internet browser to use the internet.