The Guru-Shishya parampara is perhaps the most elevating and touching characteristic of the Hindu tradition because the birthday of the great guru Vyasa is chosen for celebrating the annual festival of Guru Purnima also called the Vyasa Purnima. Guru Vyasa’s parents were Matsyagandha and Parashar Rishi. He is remembered and honored on the Guru Purnima day because his contribution to the Hindu tradition has been immensely impressive.
Ved Vyasa is the one who had classified and compiled the four Vedas into Rigveda, Yajurveda, Samaveda and Atharvaveda. He even composed the 18 Puranas and the Brahma Sutras so that the common people can understand the Vedas appropriately and get acquainted with the stories of the great saints and heroes of the Hindu culture and tradition. The credit of composing the epic Mahabharata and the Bhaagvata also goes to Guru Vyasa. Therefore you can obviously say that Ved Vyasa was the ideal and greatest guru of all times. It is to honor Guru Vyasa that the Hindus celebrate Guru Purnima.
Traditionally, the Buddhists also celebrate Guru Purnima in the months of June-July (Aashaadha month according to the Hindu calendar) because Lord Buddha for the very first time delivered and preached his 1st sermon in the deer park located in Uttar Pradesh called Sarnath. Obviously the way in which Buddhists and Hindus celebrate Guru Purnima differs but the central idea remains the same. The idea is to pay respect and offer prayers on this day to the gurus or teachers who have taught something valuable to you.
Guru Purnima is also celebrated by Jains because after Mahavira (24th Tirthankara) attained Kaivalya, he chose his 1st disciple on this day and become a guru for the very first time. In order to know more about Guru Purnima history you should talk to your elders or read up books and articles related to the festival.