We first define a datatype, and after this an abstract datatype.
The definitions given here are, to some degree, inspired by the free on-line dictionary of computing, FOLDOC at http://foldoc.org. Take a look at it!
This an the other 'Programs' are examples of specifications of abstract datatypes. A specification tells what an abstract datatype is, NOT how it is implemented.
You may skip these specifications, if you are not motivated right now...
The sample specifications of abstract datatypes illustrate a high-level and a somewhat theoretical approach