An intranet is an internal network typically hosting some service people inside the network can use.
So for example a server inside the network could host a webpage only people inside the company could see or a server inside the network could host a ticketing system only people inside the network could use.
An extranet is an external network typically hosting some service people outside the network can use.
So for example a user could log-in to a companies attendance software to clock in from home or log-in to a cloud service somewhere.