In-Ear Monitors
IEMs are devices that fit inside the ear canal entrance. 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 speakers used to produce sound in IEMs, often called "drivers," there are five common types.
Balanced Armature - Many IEMs will use multiple BA drivers to handle different frequency ranges. The advantage of BA drivers is that they're small, and thus manufacturers can fit more into a smaller space than typical designs of other drivers. The downside is that they have a very high acoustic output impedance, and as such are less efficient at producing bass vs. dynamic drivers, which makes them less tolerant of errant leaks. Many people have attributed an unpleasant tonal effect of certain IEMs using BAs to what they call "BA timbre," but this has never been proven. It is almost certain that their gripes are not to do with the driver used, but the sum total frequency response of the IEM itself at their eardrum.
Dynamic Driver - Once again, dynamic drivers are able to provide solid bass extension due to having better excursive (the ability/efficiency to move air) capability, as well as a lower acoustic output impedance—which means they also lose less bass in leaky environments. Because of this, they're often correlated to a sense of bass euphony and weight, though this isn't necessarily always true of any dynamic driver implementation. The main downside of dynamic drivers is that they're generally larger than Balanced Armatures, and thus its hard to have more than 2 or 3 of them in a single IEM at a time. However, hybrid (Dynamic Driver + Balanced Armature) setups are rather common, usually having one or two Dynamic Drivers used for bass and the rest of the Balanced Armatures producing midrange and treble.
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. Many claim these drivers provide an enhanced sense of detail, but this hasn't been tested to allow for any real conclusions to be drawn about their quality, comparatively. The major downside to Electrostatic drivers is that they can be quite a bit less sensitive than the other driver types, thus requiring more output power from amplifiers to drive sufficiently.
Planar Magnetic Driver - Recently, Planar Magnetic drivers have become more commonly used in IEMs thanks to the popularity of IEMs like the 7Hz Timeless. Planar Magnetic drivers share a trait with dynamic drivers, in that they are much more leak tolerant than Balanced Armatures. However, planar magnetic drivers used for bass tend to be quite large, such that they almost always prohibit the ability of using other drivers to shape the sound. Another downside they have is that they usually require front damping—placing a non-acoustically transparent material between driver and ear—to avoid significant treble resonances/colorations.
Piezoelectric/Bone Conduction Driver - The least common driver type, Bone Conduction purport to physically transmit sound energy through skin/bone contact, instead of transmitting sound through air. They differ significantly in their design, and are often met with skepticism regarding how efficient or noticeable their contributions to the sound profile of IEMs they inhabit are. Some designs can result in a "rattle" or a "sproing" sound when the IEM is handled, but this shouldn't be seen as an operational error. Some say they offer a unique sense of tactility and "feel" to the sound that has to be heard to be believed, ultimately it's up to the listener to decide how real/important this effect will be for them.
All of the above driver types have their own benefits and detriments, but whether or not any of these drivers is right for you is more likely down to your preferred tonal response and what kind of presentation you prefer.