The word yoga can be used to describe a series of specific methods which aid and direct the seeker towards the goal of the ending of suffering or of attaining realisation. So let me start by saying something quite obvious: all of the yogas* are practices to be performed or actions to be done. They are therefore meant to be performed by a person who thinks themselves to be a separate doer-entity.

The very existence of the (illusory) separate doer implies a duality – in fact the imagined doer is the essence of duality, the first conceptual step from which all other dualistic notions proceed from. The duality that it sets up is between that of the subject (the doer) and objects (the objects of the world in which actions are done).

The aim of all yogas is, through practice, to facilitate a seeing/realisation that the separate doer-entity is an illusion. And therein lies the apparent paradox. Yoga is action undertaken by the (imaginary) separate doer in order to see through this illusion of doership.

*Traditionally there are several key yogas outlined in the vedic texts, the main ones being Jnana Yoga (yoga of knowledge or understanding), Karma Yoga (yoga of action), Bhakti Yoga (yoga of devotion) and Raja Yoga (the king of yogas).

Also see:
How yoga works
The essence of yoga
Ramana Maharshi: The 4 paths to freedom (the 4 yogas)