Costume vs. Clothing

My costume rendering for Bramar of the Wild.

Warning: Bramar doesn't actually look anything like that guy in the drawing up there, but he does wear those clothes! My lovely illustrator, Moira, will cook me up a better likeness to Bramar (who is fantastic, by the way) very soon!

What is the difference between costume and clothing? There seems to be one, but I'm not quite sure what it is. Costume, I would venture, is meant to be visual more than functional. Clothing, on the other hand, necessitates some sort of livability. Costume is not for everyday wear, but most people wear clothing every day.

I hope that the characters in my book wear clothes over costumes. I have striven, in my musings, to give them functional, simple clothes that are a help and not a hindrance, ceremonial garb excluded. It is, I must tell you, a difficult thing to describe the complexities of clothing. There is style over function in many cases, but there is still a sense of usability, of completeness, in even an outfit as simple as jeans and a t-shirt.

There is a trap that I often fall into in endeavoring to describe everything about a character's appearance all at once. This can get pretty lengthy when you're describing clothing which such rich detail to begin with, especially when all layers of it cannot even be captured by a drawing, let alone with words.

How does one:

1. Ensure that one's character is wearing something practical that suits his needs?
2. Describe the clothing in a digestible way?

Thoughts?