An IFB carries somebody else's voice to you. An ear prompter carries your own recorded script back to you.
Same small beige earpiece, opposite direction of information. That is the whole distinction, and it is worth getting right before you agree to a job that assumes you know.
IFB
Interruptible foldback. It is a live feed from the gallery or the producer to the person on camera, and it exists so that someone can talk to you while you are broadcasting.
You hear the programme audio, and the director interrupts it to tell you to wrap, to throw to a package, to say that the guest is ready, or that you have thirty seconds. The words you say remain yours. The channel is one-way to your ear and it is carrying instructions, not lines.
This is live news and live events. Anchors, field correspondents, hosts.
Ear prompter
A recording of your script, made in advance, usually in your own voice, playing back into your ear a beat ahead of your delivery. You listen and repeat. Doing that well — the flat lay-down, the count-in, the fixed lag behind the track — is a craft in its own right.
Nobody is talking to you. There is no gallery. The only voice is the one you recorded, and the job it does is to supply words you have not memorised.
This is industrials, commercials, trade shows, corporate film and long narration. What proficiency means, and who asks for it covers that side.
Why they get confused
The hardware profile is nearly identical: a small earpiece, concealed, sometimes on a loop worn under the shirt. Both are invisible to an audience. Both let a performer work without a script in shot. To a casting director writing a breakdown quickly, or a client describing what they have seen on television, they can blur into "the thing in the ear".
The demands on the performer are almost opposites, though. An IFB asks you to keep speaking your own words while somebody talks over the top of them. An ear prompter asks you to speak words you are hearing. If you are good at one, that says very little about the other.
The third thing
There is a version that is neither: a script you hear rather than read, which responds to your delivery instead of running on regardless. Not a live voice in your ear, and not a fixed recording either.
That is what ear prompting in TellyPrompter is. The track is your own rehearsal read, and because the app is following your voice it holds when you stall and seeks forward when you skip. It takes a few goes before it feels natural, as ear prompting always does. The guide track cannot be captured on Android, and voice mode needs the Web Speech API, which Firefox does not have.
It is not an IFB and does not replace one. Nobody can talk to you through it.