What is the function and the purpose of the parts of the microscope ?

Starting at the bottom of the Compound Microscope scope working up: The base of the scope houses the illumination system and the field lens. It also provides weighted stablility.
The field lens is where the light comes out of the base. This lens focuses the light from the bulb directly into the substage condenser.
The arm of the scope supports the rest of the scope.
Directly above the field condenser is the substage condenser. The substage condenser collects the light and condenses it further into a more concentrated beam of light. With the substage condenser you can control the amount of light and to some degree the defraction of light. This is helpful in adjusting the "contrast" in the image.
Working up, you find the "stage" which is a platform to support the specimen. The stage may or may not have a specimen holder and a set of specimen holder control knobs.
The magnifying lenses are called the objectives. They look like barrels pointing down at the stage. These are usually marked 4x, 10x, 40x, and 100x. These magnifications are further multiplied by the power of the eyepieces which are usually 10x. So, when using the 4x you are actually seeing the specimen at 40x and when you are using the 10x objective you are actually seeing the specimen at 100x and so on. The 4x is used to "scan" the specimen so that you can find the most interesting area and then focus in further on that region. It is very hard to find exactly what you want to focus in on starting with the higher magnifications. Always start with the low magnifications, 4x or 10x.
The objective lenses are mounted on a rotating turret for easy selection.
Moving on up the scope is the head. It is comprised of the eye tube and the eyepiece. If you have two eyepieces your scope is referred to as a "Binocular Compound Microscope". If you have only one eyepiece, your scope is a "Monocular Compound Microscope".