When your little cousin starts naming things he wants for his birthday, counting them off on his fingers, you could say he's working on an itemization of gifts he hopes to get, or making a list of items.
You can call any kind of list, or the act of making that list, an itemization, but you're most likely to use the word if you're talking about business, money, or law. For example, if you were hired to throw a party for your cousin, you might give his parents an itemization, or an official list, of everything you spent money on, like streamers, balloons, cake, and a piñata.