IRS modify the phase and Backscatter device adjust the reflection coefficient. Backscatter device can modulate its information symbols over incident carriers by intelligently changing its load impedance and IRS can also modulate its own information. So, how they are different from each other ?
You can imagine Intelligent Reflective Surface (IRS) as a phased array but it is outside your device. A flat surface that has many reflective elements and each element's dimension is determined by the resonant frequency. You can beamform an incident wave to its desired direction by slightly changing each element's resonance. The elements resonance can be changed in multiple ways (load impedance is just one). Advantages (a) you don't need an active phase shifting mechanism, you can do beam steering with less power (b) you can provide intentional, constructive multipath to your signal, which can be used for increasing SNR. (c) you can change the phase of the reflected signal, hence it can be used as a phase modulator.
Backscatter communication is, however, harvesting the energy of your signal, enough another communication. IRS's third application can be considered backscatter. i.e you can use IRS for backscatter. But IRS originally evolved for other applications.
One difference I have noticed is that when you use low-frequency communication, (MHz range and below), the size of each unit element of an IRS becomes huge. So people don't really use it. But in that case, backscatter means you can use magnetic induction to harvest the energy and retransmit information. So I guess with respect to the frequency they are different.
Hence I would say at GHz at above frequency IRS can be used for backscatter communication. and for MHz and KHz range frequencies you use magnetic induction for backscatter.
[Also IRS is a fancier name of Reconfigurable Reflect Array (a word used by antenna designers) or a metasurface (a word used in optics). If you look for RRA's you can practical implementation papers, which I am sure would clear this confusion].
The difference is probably to do with whether the device receives the signal or just reflects it. An IRS probably is not powered by the signal it reflects, but a backscatter device (RF tag) can be, and usually is powered by the incident signal.
IRS is getting a gradually wider meaning. Many people are considering the use case when the IRS acts as a relay that support the transmission from a source to a destination, by configuring how it reflects the incident signals towards the destination.
But if the IRS is also trying to convey its own information, by encoding it into how it varies the reflection properties over time, then it also performs backscattering communications.
So the topics are overlapping. Some people might say that backscattering is a special case (or use case) of IRS technology. Others might say that IRS technology is capable of performing backscattering communication, as an add-on to its primary use cases.
You could better understand by following this paper.
Metasurface-assisted massive backscatter wireless communication with commodity Wi-Fi signal
https://www.nature.com/articles/s41467-020-17808-y
The IRS and the backscatter has the same aspects such as operating
mechanism (passive), full duplex, no RF chains, low power consumption, and low hardware cost, but they have different roles/ functionalities. The IRS is a helper that enhances the quality of transmission link by adjusting its phase and amplitude, while the backscatter is a source, which aims to transmit its own information to the receiver by riding its information on the incident wave.
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