People these days call any kind of expert a "guru," but for our purposes guru means a spiritual master, a teacher of Krishna consciousness, considered in relation to his or her disciple or student.
In the Bhagavad-gita, Krishna recommends the guru-disciple system, parampara, as the way spiritual knowledge should be passed on. He says anyone seeking the Absolute Truth should approach a guru with humility, relevant questions, and a service attitude.
A spiritual master's responsibility is to help the disciple reestablish his or her relationship with Krishna by engaging them in various kinds of devotional service, such as chanting the Hare Krishna mantra. A genuine guru presents Krishna's teachings exactly as they've been passed down through the parampara system. Anyone can become a spiritual master, provided they strictly follow their own guru and understand the science of Krishna consciousness.