In-Ear Monitors are not the same as iPod Earbuds
You may love your iPod earphones, but don't expect to get great results with them on stage.
Earphones that are designed for casual listening with your iPod (or other personal listening device) are generally not great choices for using on stage. The biggest issue is that they're not designed to block out loud ambient sounds found on most stages (even ones where everyone is using in-ears so there are no wedges) or deal with the loud transients that a live mix often brings. Often, you'll turn them up too loud to make up for the lack of isolation, and you'll lose clarity and risk serious damage to your ears. earphones just aren't going to perform very well on stage, and economizing here is not a great choice.
(The nice thing, though, is that you may find the in-ear monitors you get for playing live also sound great with your iPod, especially in a loud environment like an airplane.)
Universal Fit or Custom Molded
Universal fit in-ears (sometimes called "generic fit") have replaceable foam or soft rubber tips that adapt to the shape of your ears after you insert them. (Tip: Be sure to compress the foam before you insert them so you get the earphone nozzle properly in your ear canal and the tip can then expand in your ear to seal out ambient sound.)
An audiologist taking an ear impression that will be used to make Custom-molded in-ears place the speaker driver(s) and nozzle in a case that's molded to the exact shape of your ear.
So how do you choose? Not surprisingly, universals are, as a general rule, substantially less expensive than customs of a similar quality. Because universals can be shared, they're a great solution for churches where gear is shared among multiple ensembles and where musicians sub in and out (just be sure to keep a supply of clean foam/rubber tips on hand). Customs can provide superior comfort (especially over extended wearing), better isolation, and improved sound quality. Which is not say that universals can't sound great too. They can.
What's the difference between a universal in-ear-monitor and a custom in-ear-monitor?
Custom in-ear monitors are built specifically to your unique ear anatomy, offering a glove-like fitment resulting in unparalleled sound isolation. This option requires a trip to your local audiologist for ear impressions. Ear impressions are required for us to make your custom in-ear monitors.
universal in-ear monitors were designed to fit nearly everyone and do not require ear impressions.