A function
is a formula predifined
by Excel2003 (or by the user) that
operates with one or more values and returns a result that
will appear directly in the cell or will be used to calculate the formula it contains.
The syntax of
any function is:
name_function(argument1;argument2;...;argumentN)
Functions
follow these rules:

if the function is at the beginning of a formula, it must begin with the sign =.

Arguments and entry values are always in brackets. Do not leave any spaces after
or before each bracket sign.

Arguments can be constant values (figure or text), formulas or functions.

Arguments should be separated by semicolons ;.
Example: =SUM(A1:C8)
This
is a SUM() function that returns as result the addition of its arguments. The operator
":" identifies
a cell range
like this A1:C8 and
indicates all the cells included between cell A1 and cell C8, so the function will
be the same as:
=A1+A2+A3+A4+A5+A6+A7+A8+B1+B2+B3+B4+B5+B6+B7+B8+C1+C2+C3+C4+C5+C6+C7+C8
In
this example we can appreciate the advantage in using functions.
Formulas
may contain more than one function and functions may appear nested inside a formula.
Example: =SUM(A1:B4)/SUM(C1:D4)
There
are many types of functions, according to the type of calculation or operation they
perform. So functions can be: mathematical, trigonometrical, estatistic, financial,
text functions, date and time functions, data base functions, search and reference
functions and information functions.
To
introduce a formula you must type it in any cell, in the same way you introduce any
text,
preceded always
by the sign
=. 