Click on the animation below and you will see it turn on the LED connected to GP0 (General Purpose IN-OUT line ZERO - Pin 7):

This is what the program is doing:
In the first two lines the input/output port is set-up so that the first line (GP0) is an output. 
GP0 is turned ON and this will illuminate the LED connected to the line. The program goes to the fifth line and is sent back to the third line. Instructions 3, 4 and  form a loop and the micro constantly executes these until the power is removed.
The diagram above is very simple. The power switch is turned ON and the LED illuminates. 
You don't need a microcontroller to turn on the LED but this is the simplest circuit to explain how the microcontroller operates. 
The important point to note is the operation of the micro. It does not execute the 5 instructions in the program above, then stop. It keeps going as the oscillator inside the chip keeps operating all the time. The instruction-set for the PIC12C508A does not have a STOP or HALT instruction, so you must write the program so that the micro loops around a number of instructions. These instructions are sometimes called "do-nothing" instructions.
In the example above, the micro loops around instructions 3, 4 and 5, by executing 3, 4, 5, 3, 4, 5, 3, 4, 5, etc.  The flowchart for the micro is shown below:

USING A PIC16F84
The same program can be written for the PIC16F84 microcontroller. Although the two chips do not have exactly the same set of instructions, about 90% of the instruction-set is the same and in this course we are going to concentrate on the instructions that are common to both chips.
The instruction to access lines GP0 to GP5, for the PIC12C508A are the same as accessing RB0 to RB5 for the PIC16F84 chip.  
This means many of the programs and projects we will be presenting can be "burnt" into either chip.
"Burning" is an old name for "programming" and we use it to differentiate between the operation of writing a program - commonly called Programming, from the operation of downloading a program into a chip - also called Programming.   
As soon as a project requires more than 5 or 6 lines (input/output lines) the PIC16F84 micro will be used. 
But this is getting too advanced too quickly, getting back to the simplest program, it can be "burnt" or "downloaded " into a PIC16F84 and the circuit below will turn on a LED. 
The instruction: MOVWF 06 will turn ON (make HIGH) GP0 for a PIC12C508A or Line RB0 for a PIC16F84. 
The PIC12C508A has an internal oscillator and no external components are required. 
The PIC16F84 has some internal oscillator components and you must add the timing components (the resistor and capacitor). These two components are shown in the diagram below:

FLASHING A LED
Suppose you want to make the LED flash with an on-time of 0.1seconds and off-time=0.9seconds.  This requires a 4-step program shown in the Flowchart below:

The following animation shows the result of burning the code into a '508A: 

The program can be downloaded into a PIC16F84 and the circuit below will flash a LED:

But just before you rush out and do anything, you must understand that a PIC12c508A chip can only be burnt ONCE because the cells holding the program receive a charge during programming and this charge cannot be removed. The chip is guaranteed to hold a program for many years but you only use this chip when you are completely satisfied with a program and the project is ready for completion. 

TWO CHIPS
This course concentrates on two chips. The PIC12C508A and PIC16F84. At the moment you don't know how powerful these chips are and the type of projects they can drive. This knowledge will come "down the track" so we will explain our policy:
We firstly try to use a PIC12C508A for a project. If the project requires less than 5 input and/or output lines, the PIC12C508A will be suitable. If not, you will have to use the PIC16F84.
Programming a PIC16F84 is easy because it is programmed in the Multi Chip Programmer and placed in the project being developed. 
If the project needs a PIC12C508A, you will need an adaptor. The adaptor holds a PIC16F84 and has  oscillator components soldered to the pins. An 8-pin IC socket is soldered to the 18-pin socket and fits onto the project being developed. The PIC16F84 chip is taken out of the adaptor and "burnt" in the multi Chip Programmer. It is then placed into the adaptor and it functions exactly like a PIC12C508A chip. 
When the program works perfectly, you can burn a real PIC12C508A chip and fit it to the project. 
The diagram below shows how the adaptor holds the PIC16F84 chip and plugs into an 8-pin socket on the project you are developing.

This is the cheapest way to design a project with the PIC12C508A chip. Any other way is very expensive. 
The  PIC12C508A can only be programmed ONCE and the development of a project may take 50 or more "adjustments" until it is operating correctly. You can use 50 PIC12C508A chips or buy the EPROM version. These are very expensive and you will need at least 5 - 10 because they need to be erased for 20 minutes before a new program can be written into them and this slows down the time for development. You will also need an erasing lamp. 
Our 8-pin to 18-pin adaptor costs very little and makes developing a program very simple. 
You can program the PIC16F84 with instructions that suit the PIC12C508A and the program can be "burnt" into a PIC12C508A. 
For the example we have used our Logic Probe project. This project takes an 8-pin PIC12C508A chip and by using the 8-pin to 18-pin adaptor and a PIC16F84, a program can be developed at almost NO COST.
One clever trick is to fit a PIC chip to an IC socket and keep the two together when transferring from the programmer to a project. The chip is much easier to get out of the socket (in the project) with your fingers. See photo below:

The PIC chip is fitted into an IC socket 
for easy removal during programming.

The diagram below shows the PICF84 being burnt on the Multi Chip Programmer, fitted to the  8-pin to 18-pin adaptor and plugged into the Logic Probe project.