These are basics roli that apply to almost any language. Class is a structure that defines particular object. In Objective-C syntax class is defined by 2 separate files:
a) Header file .h
b) implementation file .m
In .h file you define an interface for this class. You make a description of class (define it's properties, it's methods, etc...). In .m file you just implement these methods. The variables that you define in .h files are called instance variables and can be accessed from outside world. The variables you define in .m file are temporary local variables.
This Wikipedia
article might give you some better understanding of the OOP terms.
Make sure you get them right otherwise it will be really hard for you to start with Cocoa programming. Make first some basic OOP examples in any language and then try with iPhone programming.
Good luck
