What is sendmail ?

[Jose Albores]

Sorry for being so "basic"!
But I'm reading lots of mssgs. about sendmail.
I read it's simple but potent, but:
What is sendmail for?
Is it for reading-sending mail as single user?
Or just for internet servers and a utility for Mail/Servers
Administrators?
TIA !

--
Jos� Albores
Buenos Aires.
Argentina.