In samsara there are two sorts of people - astika (religious) and nastika (unbeliever) - in the world of the atheist there isn't any guru.
Of the world of the astika (religious) that guru should really be called "jagadguru". Amongst the religious there are two kinds of believers. Some folk are setting their belief in a sakara (form) of Brahma and some in the nirakara (formless). The one who holds the ability to be guru to both, those who are speakers of the sakara (form) and those who are speakers of the nirakara (formless), the very same can be called a jagadguru. The meaning of this is that; of the sakara deities there are five Vedic deities, that is to say; Bhagwan Vishnu, Shiva, Shakti, Surya and Ganesha. There are different instructions for worship of all of these and for those who do not believe in the five deities he can also give instruction, the same is really a jagadguru. The person who instructs in the method of worship for devata (deities) is similar to the lowly paid vaidya (physician) who sets phials of medicine for all diseases, without also having the status of a "compounder" but calls himself a "civil surgeon". Anybody can name their own son Rama. Who can stop them then? But merely from a name he does not become Rama. Then who stops anybody writing jagadguru in front of his own name? But when you ask the mark of the jagadguru then the proper indication really is this that nobody with faith in any deity goes away from the door disappointed. Recently sampradaya (sects) have allotted a relationship of worshipping Shiva, Shakti, Vishnu etc., this is improper. In the five deities there is not any inferiority or superiority. Every deity is similarly capable of dealing with the welfare of their own devotees, however much a worshipper they are, all are vaishnava, since all deities are parts of Bhagwan. Bhagwan states that:- " GYaanaM gaNesho mama chakShurarkaMaH , shivo mamaatmaa mamashaktiraadhya . vibheda buddhayaa mayi ye bhajanti , mamaaN^gahiinaM kalayanti mandaaH .. " That is, Ganesh ji is the head of Bhagwan, Surya are the eyes of Bhagwan, Shiva is the atma of Bhagwan, Adya Bhagavati is the shakti of Bhagwan. Therefore these five gods are several equal parts, which cannot be measured and split from one another, and they are to be worshiped as equals. [If not] then worship is not being done but [instead] the cutting off of parts is being done. It is clear that denying Ganesha in desiring to be a devotee of Vishnu, but he one who is cutting off Bhagwan Vishnu's head. If any devotee of Vishnu denies Shiva then he is cutting off the atma (soul) of Bhagwan Vishnu. This is really the manner of someone denying Devi (goddess) making Bhagwan powerless. Therefore, nowadays sectarian people have ill-will and malice towards one another but call themselves vaishnava (devotees of Vishnu) or of Bhagwan Shiva, has been calling himself shaiva (a devotee of Shiva), denying Bhagwan Vishnu. They are not really shaiva and not really vaishnava, they are merely hypocrites. A vaishnava is really he who is a devotee of Bhagwan Vishnu - "vishnauratah vaishnavah" " uurdhvapuMD.hvatvaM vaishhNavatvam " `Those who possess a urdhavapund (perpendicular marking on the forehead) are said to be vaishnava.' This is not a rule. He who worships Bhagwan Vishnu, he then is really a vaishnava. But also because all deities are different parts of Bhagwan, the worshipper of any deity can be called a vaishnava. At the time one is worshipping any deity, really at that time he is a vaishnava. The whole religious world is vaishnava. Those who say that only those who have perpendicular marking on the forehead are vaishnava, and who call others non-vaishnava, are unacquainted with reality. They are a disgrace to Bhagwan Vishnu. Any devotee of Shiva or worshipper of Shakti, if they do not accept themselves as vaishnava, then they are also in error. Anyone else in samsara (worldly existence) after this manner he is not vaishnava. Those sects who voice sectarian arguments do not achieve anything for themselves nor for others either. [Shri Shankaracharya UpadeshAmrita kaNa 78 of 108] <http://www.paulmason.info/gurudev/UA_Hindi.htm#kaNa_78> translation - Paul Mason © 2006, 2007, 2009 <http://www.paulmason.info/paulmason/contactdetails.htm> Full English text of Guru Dev's 108 Discourses here: http://www.paulmason.info/gurudev/upadesh.htm