Does a class name, say UIImage+Something or UIImageView+Somethingelse, mean that it acts like a custom UIImage or UIImageView?
I think you are looking at the file names of Categories, not Classes. The plus character + is not allowed in class names or any other identifier in Objective-C.
An Objective-C category is a way of adding methods (but not instance variables) to a class you don't necessarily have the source to. For example, if you frequently want to make upside-down copies of UIImages, you can use a category to add a upsideDownImage method onto the UIImage class.
It's common to save this code in a file named UIImage+UpsideDown.m (with an accompanying header file, UIImage+UpsideDown.h).