Internet is a global network environment with multiple participants (Anybody who puts stuff on the net, such as you and me) Loads of Data/Information (Obvious) and IT.
Intranet is a network that is run by an organisation. It is usually private and allows access to only members. It is very similar to the internet but on a very small scale comparatively and is private.
Ethernet is a computer networking technology i.e. Ethernet cable. It is used in Local Area Networks. It detects errors using CSMA/CD (Carrier Sense Multiple Access/Collision Detection)