Half-duplex — walkie-talkies
On a half-duplex link, only one side can transmit at a time. If both sides try to talk at once, the signals collide and both transmissions are lost. This is exactly how a walkie-talkie works: you press the button, say your piece, say "over," and only then does the other person reply. You physically cannot both talk and clearly hear the other person at the same moment. Old hub-based Ethernet and early wireless links worked this way — and it's the entire reason collision domains and CSMA/CD (the "listen before you talk, and detect if you collided" logic) had to exist in the first place.
Full-duplex — a phone call
On a full-duplex link, both sides can transmit and receive at the exact same time, over separate transmit and receive paths (on twisted-pair Ethernet, literally separate wire pairs). This is a phone call: you can talk while the other person is talking, interrupt, and hear them simultaneously — there's no "over." Modern switched Ethernet is full-duplex by default, which is why collisions are essentially a non-issue on any current network: there's no shared medium left to collide on.
| Half-Duplex | Full-Duplex | |
|---|---|---|
| Can both sides transmit at once? | No | Yes |
| Collisions possible? | Yes | No — nothing to collide with |
| Analogy | Walkie-talkie | Phone call |
| Where you'll see it today | Old hubs, some wireless/satellite links | Virtually all modern switched Ethernet |
This is why the collision domain topic earlier called collisions "mostly historical" — full-duplex links plus switches (one collision domain per port) together removed the conditions a collision needs to happen at all.