Different people have different capacities for spiritual understanding and practice. For this reason, out of his compassion, Buddha Shakyamuni gave teachings at many levels, just as a skilful doctor administers a variety of remedies to treat different types of sick people.
For those who wish merely to attain human happiness Buddha gave teachings revealing actions and their effects, or karma; and he taught moral discipline as their main practice.
For those who wish to experience the permanent inner peace of liberation, or nirvana, for themselves alone, Buddha gave teachings on the faults of samsara; and he taught the three higher trainings – training in higher moral discipline, training in higher concentration, and training in higher wisdom – as their main practice.
For those who wish to attain the ultimate goal of full enlightenment Buddha gave teachings on the development of great compassion and bodhichitta; and he taught the six perfections – the perfections of giving, moral discipline, patience, effort, mental stabilization, and wisdom – as their main practice.
All these teachings are open to anyone who wishes to study and practice them. The experiences that are gained from practicing them are called the ‘common spiritual paths’.
Besides these teachings, Buddha also gave teachings on Tantra. These may be practiced only by those who have received Tantric empowerments. The experiences gained by practicing these teachings are called the ‘uncommon spiritual paths’.
For a comprehensive explanation of the spiritual paths of Tantra, see Tantric Grounds and Paths.