The terms "earphones" and IEMs (in-ear monitors) are often used interchangeably to describe portable listening devices with earpieces that go directly inside the ear canal. They are a counterpart to traditional headphones with ear cups that sit on or over the ear.
For our purposes, "IEMs" and "earphones" are synonymous. Some manufacturers label their in-ears as earphones, but you could just as easily call them IEMs.
Technically, the term "in-ear monitor" refers to an earphone that is meant to be used as -- and in fact was created to be -- a tool for performing artists and recording engineers. The earpieces that singers wear on stage are called 'in-ear monitors'. They provide the singer with a direct source of sound and allow them to customize their stage mix. They also allow the singer to listen to backing tracks. IEMs have slowly replaced traditional stage monitors. But earphones and IEMs are also very much enjoyed by audiophiles and music enthusiasts who enjoy the convenience and increasingly high-fidelity sound provided by this headphone type.
Both earphones and IEMs have an inherent convenience factor that traditional full-size headphones can lack, and that is portability due to their compact size. Noise isolation is another benefit. The ear pieces on IEMs and earphones go past your concha and extend into your ear canal, thanks to silicone tips attached to the nozzles. This design creates optimal noise isolation, allowing you to enjoy your music more clearly and at a lower volume. It also makes your earphones less likely to fall out of your ears. Plus, most earphone packages offer different size tips to ensure you get the best, most comfortable fit possible.
There are certainly similarities between earphones/IEMs and earbuds, most notable in appearance and the portability factor, but they are not the same thing. For starters, the ear pieces on earbuds do not sit inside of the ear canal; they sit in the space outside of the ear canal. They usually don't have tips (either silicone or foam) on them like earphones and IEMs do. Because of these factors, you get a less secure fit with earbuds. You also don't get the level of noise isolation that you get with an in-ear fit type. This means you will have to listen at higher volumes, which can damage your hearing. Additionally, earbuds are generally much less expensive and not equipped with the same types of advanced technologies as some higher-end earphones and IEMs. You are not going to find multiple driver types in earbuds, for example.
IEMs come in both universal and custom styles. As the name suggests, universal IEMs are designed to fit most ear canals. Most come with multiple sizes of ear tips to fit on the nozzle of each ear piece. Finding the right tip for your IEM is critical to fit and comfort. If you don't find what you need included with your IEMs, there are plenty of sources great tips out there, including SpinFit silicone ear tips (spinfiteartip.com) and Comply foam tips (complyfoam.com).
Custom IEMs, on the the other hand, are designed for a single user's unique ear canal. You must obtain an impression from an audiologist. Custom IEMs provide the user with a personalized fit for optimum comfort and security. Additionally, you will get unrivaled sound isolation with this fit type. Be aware that custom IEMs will cost a little more, and you won't be able to re-sell them because they are specifically made to fit your unique ear canal.
In-ear monitors were initially created for use in the music studio and on stage. In fact, the first pair of custom IEMs was designed by Jerry Harvey for drummer Alex Van Halen, who was experiencing some hearing loss as a result of traditional stage monitors. He also needed a way to isolate his own sound from the ambient noises inherent in large arenas and to improve communication with his band mates. This was in the mid-1990s, when in-ear monitors were not used in touring to the extent that they are today. While they have practical, professional applications, IEMs also enjoy a loyal fan base of audiophiles and music enthusiasts. IEMs are great for commuting, traveling, running errands, the office, and home listening sessions.
For starters, earphones and in-ear monitors are the most portable headphone type. Small enough to fit in your pocket and discreet enough to wear without advertising that you're using them, IEMs are easy to wear while shopping, exercising, traveling, and more. Another advantage is that IEMs can easily be worn during exercise, even high-impact activities like running. Because they sit snugly in the ear canal, you can be assured they will stay in place during a good amount of movement and jostling. Of course, for the most secure fit, you want to look at custom IEMs.
This is very much subjective. We all hear differently and we all have personal preferences and practical considerations when choosing one over the other. That being said, IEMs generally won't give you the soundstage or bass of a full-size headphone. However, many manufacturers of higher-end IEMs have come out with models that feature multiple driver types and sophisticated technologies that give listeners a high-end listening experience that rivals the performance of traditional headphone styles. With IEMs, you get a highly isolated listening experience due to their incredible sound isolation. Because you don't get as much ambient, outside noise, you can listen at lower volumes, which can protect your hearing and reduces ear fatigue. You also get a lot of detail with IEMs; however, bass can be lacking.
In-ear monitors can contain one or as many as 12 drivers per earpiece. Dynamic drivers are the most common and least expensive IEM driver type. Able to cover the entire frequency range, they are known for great bass response in your music. Some high-end IEMs utilize electrostatic drivers (known for detail and treble extension) and planar magnetic drivers (known for deep bass response). An additional driver type called balanced armature is exclusive to IEMs. Originally designed for hearing aids, the tiny size of balanced armatures make them ideal for IEMs. Balanced armatures are not known for having a wide frequency response -- they are better for mids and highs -- so they are used in combination with other driver types.
Technically, no. You can drive most IEMs directly from your phone or computer if you want to. But for higher-sensitivity IEMs, in particular, you'll want to pay attention to your source -- a quiet source will be key for reducing noise. And really, for higher-end IEMs in general, you'll want to make sure that you are maximizing the sound quality. For better sound on the go, you might look at using a portable DAC (digital to analog converter) with your phone, or a portable DAP (digital audio player) as your source.