Due to the vast number of Items, they must now be divided into separate classes, six to be exact. Each class represents the rarity and in most cases, power and usefulness of that item. Classes don't do much in themselves, but they serve as an easy way to organize your items by power, and also they mostly serve for Random-Crafting. So, anyway, the classes are:
Usual Items[]
The Items of the Usual class are so common that they appears on most bloons of the same kind. (For example: Ores and Rubber are all Usual Items, each Ore appears after you pop pretty much any bloon of that kind.) Items crafted from this class of material often requires a ridiculous amount of Usual Items to craft. (For example: an Auracryst needs as much as 750 Aurae Dusts!)
Common Items[]
The items of this class are still fairly common and spawns a lot, but unlike Usual Items, they don't appear on pretty much any bloon you pop. In fact, you'll come across 1 to 3 Common Items each round on average. Many items of this class are still Loot Items and Road Items though.
Uncommon Items[]
These items are somewhat hard to come by (once every 1-5 rounds), but you can still get a good amount each game.
Rare Items[]
You'll usually need to pass many rounds to come by 1 Item of this class. These items are usually used to craft strong and formidable Equip Items (these crafted items are usually of the Epic class below.) Also, there're really few Loot Items and Road Items in this class, while more Equip Items appear. (Equip Items tend to be rarer and higher-classed than Loot Items and Road Items, because they can be used permanently.)
Epic Items[]
In most cases, you'll never come across an item of this class just by playing. These items are crafted from many Rare Items, and need a large effort to find all the ingredients to create one. Almost all of these items are really, really strong Equip Items. In fact, they're among the most powerful items in the game!
Ultimate Items[]
You'll never get one of these items by normal means. These items are extremely, extremely rare. In fact, they're so rare that they're often one-off in the whole game and even affects the storyline in some ways! An example of Ultimate Items are the parts of the ΩΧ.