What's the difference between an extranet and a VPN, and what common uses does each one serve? What technology stacks are often use to implement each?
Extranet is outside the firewall, private not public (DMZ), but only open to certain people/companies/oranizations.... https, instead of a vpn client connection.
Intranet is inside the firewall, and private, sealed off from the internet. You can use VPN to access the intranet securely though.
They are names for very different concepts.
A VPN is a "virtual private network", a piece of software that creates an encrypted communication between two (potentially) far-away computers such that nobody in between can see the contents of the communication.
An extranet is something of a made-up term, referring to a web site that a company publishes for the benefit of its vendors, partners, or customers. The name "extranet" was derived from "intranet", which is a web site that's intended for use only within a company. That term itself was sort of perverted from "Internet", which is literally a network of networks (web sites came around long after the Internet per se was created).
So an extranet is really a web site with a specific purpose. It's not a technology for setting up communications between computers like a VPN is.