The word "Khalsa" means "pure", Khalsa's are Sikhs which have undergone the sacred Amrit Ceremony initiated by the 10th Sikh Guru, Guru Gobind Singh.
The Khalsa order was initially created on Baisakhi Day March 30, 1699, with Guru Gobind Singh baptizing 5 Sikhs and then in turn asking the five Khalsa's to baptize him.
These five are revered in Sikhism as the "Panj Piyare", or the "Five Pure/Beloved Ones".
Since that historic day, the tradition has continued, as a group of Five baptized Sikhs (Panj Piyare) initiates others who wish to take up the "Five Ks" and the other requirements, and join the order of the Khalsa.