In Sanskrit the word Yuj stands for union. Yoga is formed by combination of planets or by placement of one planet in special status. There are two kinds of Yogas. They are the shubha or auspicious yogas and the other is Ashubh or inauspicious which is also known as Ava yogas.
There are thousands of Yogas described in the classical Books of Astrology. These may be classified broadly as Dhana Yogas ( yogas giving wealth) , Raja yogas which give power, Kirti yogas which give name and fame, Sanyasa yogas found in the horoscopes of saints, Putra yogas (yoga for children), Sangeeta Vidhya yogas found in the horoscope of all musicians, and Vahana yogas for vehicles etc.
In many horoscopes the Yogas overlap each other and if they are all good yogas the person leads royal life. Similarly there are Ava yogas bounding in many horoscopes. These are the sufferers of this earth who have earned their own suffering by bad deed in the past lives.
The yogas are great teachers. They tell us what ever you have earned by your own merit , you are enjoying them and what ever demerit was there you are suffering for them. So don’t fret and fume, don’t be jealous. You cannot forcibly take away any one’s wealth name or fame. Always try to do well and be good. In short “as you sow, so you will reap”
There are very many rules for the formation of yogas. Mere presence of two or more planets in a sign cannot form a yoga.