Guru Teg Bahadur became the Sikh leader in 1665 and led the Sikhs until 1675. Teg Bahadur was executed by Aurangzeb for helping to protect Hindus, after a delegation of Kashmiris came to him for help when the Emperor condemned them to death for failing to convert to Islam.
Guru Gobind Singh exemplified the teachings of Sikh principles during his lifetime. He not only taught the principles of the faith, but also felt the pain of sacrifice in upholding those beliefs. Guru Gobind Singh lost his father, mother and all four sons to a religious war being waged by emperor Aurangzeb, the last of the six grand Mughals. Aurangzeb demanded that all Sikhs either convert to Islam or be killed. It was in this fight for spiritual survival (or as Guru Sahib himself called it, Dharamyudh, the war for righteousness) that Guru Gobind Singh felt the pain of loss.
Guru Gobind's father, Guru Teg Bahadur, was beheaded in Delhi (where the Gurdwara Sis Ganj Sahib stands today) on the orders of Aurangzeb for refusing to convert to Islam. Guru Gobind Singh's two elder sons, Sahibzada Ajit Singh and Sahibzada Jujhar Singh, died fighting Mughal troops at Chamkaur Sahib. His mother Mata Gujri was killed along with his younger sons Sahibzada Zorawar Singh and Sahibzada Fateh Singh for refusing to convert to Islam.
Guru Gobind Singh also finalized many elements of Sikhism. He proclaimed himself the last living Guru and established the Guru Granth Sahib as the eternal Guru of the Sikhs. This elevates the reverence of Granth to that of a living Guru. However, it was only spiritual authority that was bestowed upon the Guru Granth. Guru Gobind also compiled the Dasam Granth sometimes known as the Dasven Paatshah Ka Granth (the Holy Book of the Tenth Emperor). He also established the Sikh baptism ceremony, the partaking of amrit or Amrit Sanskar. He was a great warrior, leader and poet -a rare combination.
I have a female friend who told me the one thing her Sikh parents strictly forbade her to do was date a Muslim. ;)