Here we are going to have a look at a basic unit.

Units are modules which are parts of your program, an example is that you can have 20 units in one program, or even more if you need them.

Another example is in a quiz program a unit can hold all the questions and answers, another unit can be for comments made in the program, in other words you can break down your program into logical units.
If your program is large these units make it easier for you to remember where you have put each part of the program, as the example above shows, you can easily find all the questions for your quiz program because they are all in the
questions unit.

When you start a new project in Delphi you will find that Delphi will automatically create a program module and unit that defines the main form, Form1.

If you decide you want to add a new form, maybe an AboutBox then Delphi will create a new unit for that AboutBox and any other new forms you add to your program.

Knowing what a unit is and how its put together will help you become a better Delphi programmer.

A Delphi unit has the file extension .pas, so unit1 will have a pascal file called unit1.pas.
The program files have the extension of .dfm.

Here is a basic unit:

==============================================================
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
  TForm1 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

end.
==============================================================

Now lets break it down and have a look at the parts to a unit and what they mean.

First we have the unit keyword
------------
unit Unit1;
------------
Next to the unit keyword is the name of the unit, (in this case the name of the unit is Unit1.pas) and next to the unit keyword you see the word Unit1 which represents the Unit1.pas file.
The name next to the word "unit" must be the same as the name of the unit otherwise an error message will show.
So if your unit is called quest1.pas then you must have
------------
unit Quest1;
------------
at the top of that unit.

Next we have the interface section.
The interface section starts at the interface word in the unit and finishes at the implementation statement, so everything between interface and implementation is part of the interface section.
Everything that in the interface section is visible to any units that use this unit.

The Uses clause, this is where units that your program is dependent on are listed.
You can see the standard ones here:
-------------
uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;
-------------
When you add a component to the form (Form1) you can then see Delphi has added the unit for that component here in the Uses clause.
It is not often that you will want to add your own units names here because you  can add them after the implementation statement in your own Uses clause, you will see many examples of this when you view the source code of many
Delphi projects.

Type Clause:
This is where the Type declaration of the form begins.
When you start a new project, Delphi declares a new class type for the form, this new class type is TForm, which is derived from type TForm which is also a class.
-------------
type
  TForm1 = class(TForm)
-------------

If you add a component to your form (Form1) then you will notice that the source code in the unit changes like this (we will add a Memo):
-------------
type
  TForm1 = class(TForm)
    Memo1: TMemo;
-------------

Should you decide to change the name of the memo to FirstMemo using the Object Inspector, then you will see this change in the Delphi Code Editor:
-------------
type
  TForm1 = class(TForm)
    FirstMemo: TMemo;
-------------

Notice how Delphi makes these changes for you.
You can use the Object Inspector to make changes such as changing the name of a component.

The Implementation Section:
This is everything between the implementation statement and either the finalization statement or the end of the pascal unit file.
The finalization section is optional and the code in the finalization section is executed upon program shutdown.