What is the purpose of Karma Yoga. What does it do? Is it compulsory for everyone who is worker to do it? Can any work be done as karma yoga?
Yes, selfless work that is done for someone else like charity, Social service, NGO's, fighting for social cause etc.,, is regarded as Karma Yoga. In short, anything good that is not done for one's sensual pleasures, but for society.
Bhagavad Gita 5.7 The karma yogis, who are of purified intellect, and who control the mind and senses, see the Soul of all souls in every living being. Though performing all kinds of actions, they are never entangled.
Bhagavad Gita 5.8 – 5.9 Those steadfast in this karm yog, always think, “I am not the doer,” even while engaged in seeing, hearing, touching, smelling, moving, sleeping, breathing, speaking, excreting, and grasping, and opening or closing the eyes. With the light of divine knowledge, they see that it is only the material senses that are moving amongst their objects.
Explanation of the importance of Karma Yoga by Swami Vivekananda
It is in love that religion exists and not in ceremony, in the pure and sincere love in the heart. Unless a man is pure in body and mind, his coming into a temple and worshipping Shiva is useless. The prayers of those that are pure in mind and body will be answered by Shiva, and those that are impure and yet try to teach religion to others will fail in the end. External worship is only a symbol of internal worship; but internal worship and purity are the real things. Without them, external worship would be of no avail. Therefore you must all try to remember this.
People have become so degraded in this Kali Yuga that they think they can do anything, and then they can go to a holy place, and their sins will be forgiven. If a man goes with an impure mind into a temple, he adds to the sins that he had already, and goes home a worse man than when he left it.
This is the gist of all worship — to be pure and to do good to others. He who sees Shiva in the poor, in the weak, and in the diseased, really worships Shiva; and if he sees Shiva only in the image, his worship is but preliminary. He who has served and helped one poor man seeing Shiva in him, without thinking of his caste, or creed, or race, or anything, with him Shiva is more pleased than with the man who sees Him only in temples. A rich man had a garden and two gardeners. One of these gardeners was very lazy and did not work; but when the owner came to the garden, the lazy man would get up and fold his arms and say, "How beautiful is the face of my master", and dance before him. The other gardener would not talk much, but would work hard, and produce all sorts of fruits and vegetables which he would carry on his head to his master who lived a long way off. Of these two gardeners, which would be the more beloved of his master? Shiva is that master, and this world is His garden, and there are two sorts of gardeners here; the one who is lazy, hypocritical, and does nothing, only talking about Shiva's beautiful eyes and nose and other features; and the other, who is taking care of Shiva's children, all those that are poor and weak, all animals, and all His creation. Which of these would be the more beloved of Shiva? Certainly he that serves His children. > He who wants to serve the father must serve the children first. He who wants to serve Shiva must serve His children — must serve all creatures in this world first. It is said in the Shâstra that those who serve the servants of God are His greatest servants. So you will bear this in mind.
Let me tell you again that you must be pure and help any one who comes to you, as much as lies in your power. And this is good Karma. By the power of this, the heart becomes pure (Chitta-shuddhi), and then Shiva who is residing in every one will become manifest. He is always in the heart of every one. If there is dirt and dust on a mirror, we cannot see our image. So ignorance and wickedness are the dirt and dust that are on the mirror of our hearts. Selfishness is the chief sin, thinking of ourselves first. He who thinks, "I will eat first, I will have more money than others, and I will possess everything", he who thinks, "I will get to heaven before others I will get Mukti before others" is the selfish man. The unselfish man says, "I will be last, I do not care to go to heaven, I will even go to hell if by doing so I can help my brothers." This unselfishness is the test of religion. He who has more of this unselfishness is more spiritual and nearer to Shiva. Whether he is learned or ignorant, he is nearer to Shiva than anybody else, whether he knows it or not. And if a man is selfish, even though he has visited all the temples, seen all the places of pilgrimage, and painted himself like a leopard, he is still further off from Shiva.