We have mentioned many times that Maple makes a distinction between expressions and functions. Expressions are what computer scientists call data structures and functions are what they call procedures. To put it very briefly, a data structure is a collection of information and a procedure is a collection of instructions. In order to explain this difference better, in this and the next chapter we look at some elementary examples of data structures and procedures in Maple. To really understand what makes expressions and functions different in Maple we need to get a sense of what data structures and procedures are. This section is a very basic introduction to how Maple handles data structures.