about classes

class:A class is a software construct that defines the data and methods of the specific concrete objects that are subsequently constructed from that class.
The data represents the state of the object.
The methods represent the behavior of the object.

for more information about class: http://msdn.microsoft.com/en-us/library/aa868834.aspx

Variables are the data for the class. Variables in an X++ class are specific to objects that are constructed from that class. Every object constructed from the class declaration has its own copy of the variables. Such variables are known as instance variables. You cannot declare static variables in X++.
Methods:Methods are the sequences of statements that operate on the data. Methods are typically declared to operate on the instance variables of the class, and are known as instance methods or object methods. In X++ you can also declare static methods.

for more information about method : http://msdn.microsoft.com/en-us/library/aa673265.aspx
How to create a class in AOT:  http://msdn.microsoft.com/en-us/library/aa631180.aspx

Types of classes:There are two types of classes
1.Application classes.
2.System classes or kernel classes.

1.Application classes are implemented in X++.
They are available in the Classes node in the AOT.

 for more information about all application classes:
 http://msdn.microsoft.com/en-us/library/aa573512.aspx

2.System classes, sometimes called kernel classes, are implemented in C++.
They are listed under the AOT >System Documentation > Classes node .
However, the source code for these classes is not available.
for more information about all system classes:
http://msdn.microsoft.com/en-us/library/aa853470.aspx

Methods in X++ :

http://msdn.microsoft.com/en-us/library/aa673265.aspx