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Sikhism is a Dharmic religion, originating from the Punjab region, nowadays split between the countries of India and Pakistan. Sikhism is a monotheistic religion, meaning they believe in only one God (Ik Onkar in Punjabi) and came to be through guidance of the ten Gurus (leading teacher of Sikhism) with the first Guru, Guru Nanak, founding the religion.

Five Ks

The Five Ks (Panj Kakkar) are the five objects/acts of faith that all baptized Sikhs must follow at all times. They are Kesh (Uncut hair) which represents the acceptance of God's will and often tied into a topknot and covered with a turban,

Kangha is a small wooden comb which symbolizes one's clealiness and discipline and used to keep hair neast and tidy

Kara is a steel bracelet which stands for eternity, strength and is a reminder of staying righteous. Wearing it on the rist is also a symbol of one's connection to God.

Kachera meaning cotton undergarment which historically made it easier to move in battle but is still worn today as a symbol of chasity, self-discipline and moral restraint.

Lastly, the Kirpan or the ceremonial sword/dagger that should NEVER be used in offense and only serve for self-defense or protecting others from harm when peaceful resolution doesn't work.

Why so nice?

You might have met a Sikh and might have found them to be generous and selfless. True Sikhs strive to live by the principles of, as stated by Guru Nanak Dev Ji, equality, compassion and sharing. All of the Gurus taught that wealth should be used to help others in need, not just for one's personal gain. Many Sikhs also practice Dasvandh, where one donate 10% of their earnings to charity and Sewa, aiding others without expecting something in return.

Gurus

The first Guru, Guru Nanak, established Sikhism as a religion and himself and religious leader (Guru). Gurus were chosen based on their divine wisdom, merit and whoever the former Guru wished to succeed him. Though, Guru Gobind Singh, the tenth Guru, feared that the human ego could corrupt Sikh teachings, thus leading him to declare the Guru Granth Sahib (a book with a collection of hymns, teachings and poems seen as the eternal truths of Sikhism) as the next Guru.

Guru Gobind on his deathbed formally installed the book as the eternal Guru by saying that all Sikhs are commanded to accept the Granth as their Guru.

Lack of hierarchy

Since Gurus hold the authority of God and the Guru title being handed over to the Granth Sahib, Sikhism doesn't have a human religious leader, unlike say Catholicism's Pope. In Sikhism, any Sikh can perform communal (and personal) rituals.

Note that the Gurdwara (Sikhism's house of worship) does not have a single head but does have the Granthi, a volunteer who is responsible for reciting scripture and organizing services but have no other authority. Gurdwaras are led democratically by the Sangat which is the community of people who meet and worship there.