differences between internet,intranet and extranet

internet is a network which is for public, world where intranet and extranet is a network for private use,
internet is a network where anybody can access, from around the world, but in intranet and extranet we add some privacy so only allowed people can access.

there is no need for example of internet, hope u know!!
lets give an example of intranet and extranet.
we sometimes login some websites which requires our details to login, such as bank to view our bank statement, isp, university, so here we r connecting to extranet, bank allow us to access their internal network. In intranet internal users are allowed to use, such staff of bank, isp or other company which allowed their internal staff to use their network and no outsiders allowed in this case.
hope it is alright!