Although not all earphone and in-ear monitor aficionados would agree on this point, many would say that the most highly-evolved of all in-ear listening devices are true "custom-fit in-ear monitors"- or "in-ear monitors," for short.
Devices in this category were first created for use as very-high-performance, personal music-monitoring systems intended for professional musicians performing on stage. Several factors drove musicians (and their sound engineers) in this direction. First, loudspeaker-type stage monitors were large, bulky, feedback-prone, and tended to "beam," meaning performers had to stand in certain "sweet spots" on stage in order to hear themselves sing and play. But perhaps the biggest drawback was that speaker-type stage monitors had to play extremely loudly to be heard above the din of onstage amplifiers, instruments, PA systems, etc. Obviously, the last thing musicians needed was to have their hearing damaged by their own monitors.
In contrast, personalized, custom-fit in-ear monitors offered a host of benefits. Each singer and player on stage could have his or her own small, unobtrusive custom-fitted monitor that offered very accurate monitoring-grade sound, was compact and thus allowed freedom of movement on stage, and offered a tremendous amount of noise isolation so that sensible, non-damaging playback levels could be used. Finally, since the earpieces of custom-fit in-ear monitors were typically larger in cubic volume than those of universal-fit earphones, there was enough room inside for designers to install sophisticated arrays of miniature drivers for even higher sonic performance.
The key to the whole custom-fit in-ear monitor concept revolves around the ability to take custom ear-mold impressions (a service typically provided by qualified professional audiologists) and then build earpiece housings for monitors that are an exact fit for the wearer's ears. The upside is that the resulting monitors give a truly optimized fit for their owners, but the downside is that the monitors fit one person only (for obvious reasons, there's no "universal-fit" aspect to custom-fit in-ear monitors).
Over time, what started out as an answer to the needs of working musicians turned out to be a great high-performance personal music-monitoring option for professional sound and recording engineers, record producers, and, naturally, for serious audiophiles. Three things-all of them desirable-set custom- fit in-ear monitors apart from their universal-fit earphone counterparts: superior noise isolation, superior comfort and fit (especially for those with hard-to-fit ear shapes), and-at least in principle-superior sound quality.
Note: Certain makers of universal-fit earphones have begun to call their products "in-ear monitors," and a few may be capable of delivering the sound quality to back up their claims. Even so, I would suggest that custom-fitted earpieces are one of the defining characteristics of true in-ear monitors. Without the superior noise isolation that custom-fit earpieces provide, universal-fit earphones typically allow too much noise leakage
for on-stage use (or for use in other environments where noise may be present). Audiophiles can verify this for themselves by comparing the performance of universal-fit earphones vis-a-vis custom-fit in-ear monitors when traveling, say, on jet aircraft. Having performed such comparisons many times, I can vouch for the fact that the superior noise isolation of custom-fit in-ear monitors makes them the creme de la creme for on-the-go listening.