What is the difference between "Odzież" and "Ubrania"?
Basically synonyms but there is a slight stylistic difference. In informal conversation odzież might sound a bit artificial, you would rather use ubranie or ubrania. It's roughly like clothes and garments. If you want to sound very Polish use the word "ciuchy", e.g. muszę kupić nowe ciuchy. Obviously in informal conversation.
In the terms of meaning those are pretty much synonyms. However grammatically they are totally different. While ubrania is a plural form of ubranie and it can roughly be described as a set of clothes to be worn together (that's not so simple though), odzież is a singular noun that doesn't have a plural form and is a name of a category of all things which are worn as a coverage to our bodies (except shoes and maybe hats).
In some idioms one of the words might sound awkward. For example odzież wierzchnia is perfectly fine, ubranie wierzchnie is a bit strange.
I'd say that it's "ubranie" that is 'one set of clothes'... unless I misunderstood you. "ubrania" are, well, "clothes".
"odzież"... to me it's like the general notion of "clothing". I would never ever say that there is "odzież" on my bed or in my wardrobe. But then a shop selling clothes is "sklep odzieżowy". You also have "odzież ochronna" for "protective clothes"...
@iJany: shortly, I'd say that you will rarely have a reason to use the word "odzież".
Yes, I was referring to a singular form as a single set of clothing. Sorry for being unclear. I'd use both are in use in a slightly different context, one just has to be careful about the grammatical form to use.
As they said. Basically the rule of thumb is:
Odzież - clothing
ubrania - clothes
There are shops that sell imported, second-hand clothes here and often they're named something funny like "Odzież zachodnia" or "Odzież skandynawska"