A phone that is software instead of hardware. Ours is the browser, which is a deliberate choice and not everyone's preference.
An application that does what a desk phone does — dial, answer, hold, transfer — over the internet rather than a phone line. It may be a desktop app, a mobile app, or a browser tab.
The audio path is the same in all three. What differs is what has to be installed and who has to maintain it.
It is worth being fair about this. A native app can hold a call through a browser crash, integrate with headset buttons more reliably, and keep running when somebody closes the wrong tab.
Teams on very high volume with proper headsets often prefer one, and that is a reasonable preference.
Because the alternative is an install on every machine, an update cycle, and a support queue full of version mismatches — for a business that may not have anybody whose job is IT.
Tell us what you need — education, study abroad, healthcare, property, technology, marketing, recruitment or outsourcing — and the right team will come back to you.