Variables

Some examples:

var I : Integer; S : String;

var I1, I2, I3 : Integer;
 

Object Pascal requires you to declare all variables before they are used and everytime you declare a variable you must specify a data type.

Example:
 

procedure TForm1.Button1Click(Sender: TObject);
var I : Integer;
    A, B : Char;
    Finished : Boolean;

begin
    I := 0;
    //Some code here
    Finished := True;
end;

You will find that you cannot mix two different data types.

Using the example above, I := True;   will not work.

These variables both have different data types, doing this will result
in a compiler error: Incompatable types: 'Integer' and 'Boolean'.

You can change Variables during program execution like this:
 

procedure TForm1.Button1Click(Sender: TObject);
var I : Integer; S : String;
begin
    I := 0;
    S := 'Chuck Norris - Karate.';
    I := 10;
    S := 'Walker Texas Ranger.';
end;

The above code doesn't really makes sense, but you can see that the variables
I and S are changed as the code is executed.