IEMs, or more commonly known as In ear monitors, are specifally designed earphones that alot of live musicians use when conducting a concert or just any sort of live playing. These earphones are often custom moulded to the musician’s ears, so that a minimal amount of sound passes through the earphones to distract the musician from what he/she is supposed to hear.
Studio Monitors are generally used in professional studios, in which a flat frequency needs to be played from the speakers for the musician to hear. This means an accurate representation of a mix can be transcribed into the monitors without having any alterations to it. This allows producers and engineers to mix and master a track way easier then using let’s say a home surround sound systems.
IEMs are not commonly used in a studio environment as it’s very difficult to determine the stereo space on earphones. Things like reverbs and panning are much more challenging to picture and mix without having studio monitors to guide our ears. In ears are often way to distracting as the sound is going in directly to our ears, which is why it’s much more suitable for live usage.