Computer - a building.
Internet - the world outside the building.
Internet (or network) connection - a door into the building.
World Wide Web - all the roads and sidewalks in the world.
web browser - a magic car with a chauffeur and instant transportation to most places in the world.
website - a store, a library, etc., that can be visited by car.
cookie - a stamp printed on you when you visit a store, etc., used to identify you as a repeat customer, and possibly profile you.
email - a package. Packages can be sent almost instantaneously to anyone in the world for free.
spam - an unsolicited package from a direct-marketing company.
program - a person.
firewall - a security guard at a door into a building who can alert the owner when unauthorized persons attempt to enter or leave, and prevent them from doing so.
port scan - the turning of doorknobs on a building by a person to see if the doors will open.
anti-virus program - a security guard trained to recognize disguises, etc., and arrest and/or kill(!) sneaky strangers.
key logger - a spy who can secretly record all actions that occur inside a building.
file - an object, such as a music recording, a movie, a car, a person, etc. Perfect copies of objects can (magically) be made for free.
file-sharing network - a place where gods meet to allow others to (magically) perfectly duplicate any objects they bring along, and keep the duplicates.
symmetric encryption - use of a trunk with a lock that requires the same key to lock and unlock it.
secret key - a key used to lock and unlock the trunk.
asymmetric encryption - use of a trunk with a special lock that requires a pair of keys: when it is locked with one, it can only be unlocked using a different, related key.
public key - one of a pair of keys (call it "A") to a special lock on a trunk. Copies of this key are made widely available.
private key - the other of a pair of keys (call it "B") to the special lock on a trunk. Copies of this key are kept private.
message digest - a person's fingerprint. Fingerprints can also (magically) be made of any type of object.
digital signature - an object's fingerprint locked in a small trunk with special lock using key "B". It is sent along with the object. The object's authenticity can be verified by anyone with a copy of key "A" by unlocking the trunk and confirming that the object's fingerprint agrees with the one that was locked in the trunk. (Only someone with a copy of key "B" could have locked it in the trunk.)
Editorial Note: Analogies added to this article should be consistent with those already here. Otherwise, other self-consistent sets of analogies should be appended to the article.