There are four yogas: the three listed in the Bhagavad Gita and raja yoga. The three from the Bhagavad Gita are jnana, bhakti and karma. Jnana is the path of knowledge, bhakti is the path of devotion, raja is the path of meditation and karma is the path of selfless service.
Karma is a Sanskrit word that means action. In karma yoga, one pursues one’s duty selflessly without attachment to the outcome of her or his actions. This purifies one’s mind and leads to moksha (liberation).
For more information, you can consult the Bhagavad Gita or the Upanishads.