Showpieces are just what they sound like: for show and not much else. They are generally awkward and cumbersome in some way, I require a handler to wear them at all times, usually because I have little to no dexterity with my hands, poor vision, no ability to remove or or put pieces back on without help, or all of the above.
I don’t like making these types of costumes on commission unless my client fully understands the limitations and difficulties surrounding the design of the character or creature they want built.
None of this is meant to deter or detract from the possible fun of wearing the costume, though. Despite the often many difficulties that come from wearing them, it’s usually because of these obvious difficulties that it’s fun to interact with others. There’s generally an understanding between a cosplayer and the public when it comes to very detailed or highly advanced pieces; they tend to know it’s obviously a lot of work to wear such a thing, and yet you’re doing it for entertainment purposes, to show your work, or both (for me it’s definitely both; I love making people smile with my work!) That in and of itself makes the experience very rewarding, along with being able to discuss build information and other questions that more than likely will arise from such eye-catching costumes.

Functional costumes I classify as slightly more complicated than what a “casual” costume would be, but much easier to wear than showpieces. They still contain some interesting effects or details, but because of the style they’re usually easier for the wearer to deal with. A good example of this classification are standard fursuits. You’ll still want to take breaks every now and then (usually for heat control), but they’re wearable for a decent amount of time and pieces can (usually) be easily taken off or on with little hassle. Sometimes their dexterity and visibility can be decent or even good. Handlers aren’t necessarily required (though it’s usually a good idea to have some around just in case). All in all, for newbie cosplayers they might take some getting used to but at a certain point they usually become comfortable and fun to wear (and interact with others of course)!

Casual costumes are completely or almost completely non-intensive for the wearer. Most general cosplays fall under this category because they usually consist simply of outfits (which can still be insanely detailed!) and maybe a prop or two, but little or none of the costume actually impact the wearer’s movement, visibilty, etc.