IEMs are devices that fit inside the ear. You can think of this as the natural progression from the well-known 'earbud'. In-ear monitors can vary in size, shape, and degree of insertion into the ear canal, however they generally create a seal with the ear canal. IEMs can also be fitted with different tips that vary in material type from silicone to foam, along with different sizes to best fit different ear canal shapes. For the transducers, there are three common types.
Balanced Armature - Usually high performance IEMs will use multiple BA drivers to handle different frequency ranges. The advantage of BA drivers is that they're able to provide incredible detail for treble and midrange frequencies. The downside is that they aren't as capable for bass frequencies as dynamic drivers are. BA drivers also occasionally impart what some people call the "BA timbre", and this is a kind of metallic smearing effect - however not everyone notices this.
Dynamic Driver - Once again, dynamic drivers are able to provide solid bass impact due to high excursive capabilities (the air can physically be moved with increased force).
Electrostatic Driver - Occasionally electrostatic drivers are used in IEMs, but they're generally reserved only for high frequencies, leaving the rest of the frequency range up to BA or dynamic drivers.
While we often see pure multi-balanced armature driver IEMs, they also commonly use a hybrid system with balanced armature and dynamic drivers, and occasionally the use of electrostatic drivers. While it's possible for single driver IEMs to exist, they're not as common as ones that use multiple drivers. The reason for this is because certain transducer types are best suited to different frequencies, and in the case of hybrid IEMs, dynamic drivers will handle the bass frequencies, while the balanced armature drivers handle the rest of the frequency range.