Satguru, or sadguru, means the true guru. However the term is distinguished from other forms of gurus, such as musical instructors, scriptural teachers, parents, and so on. The satguru is a title given specifically only to an enlightened rishi/sant whose life's purpose is to guide initiated shishya along the spiritual path, the summation of which is the realization of the Self through realization of God, who is omnipresent. A Satguru has some special characteristics that are not found in any other types of Spiritual Guru. The words 'Sant' and 'Satguru' firstly came into existence from the spiritual ideology of “Sant Samrat Satguru Kabir Sahib” in the 15th century. Kabir Sahib says "Satpurush Ko Jansi, Tiska Satguru Naam|" meaning the one who has seen the supreme lord of truth- Satya Purush is Satguru. "Devi dewal jagat mein, kotik poojey koye. Satguru ki pooja kiye, sabb ki pooja hoye". Kabir Sahib says Worship of Satguru includes in it worship of all deities.
Example via www.ramdass.org: The Need for a Guru