It is to attribute Allah's qualities to someone else. This is called Shirk because Allah's qualities are unequaled by any of His creatures, be they angels, prophets, Auliya, martyrs, Imam or other devoted person. Their qualities cannot match that of Allah.

Shirk fil Qudrat, Attributing Allah's quality of power to anybody else.

To believe that a certain Wali or martyr can bring about rains, fulfil desire or cause the birth of babies. Or to kill or bring anything into life or to bring benefit or damage is in their power.

Shirk fil 'Ilm, attributing Allah's power of knowledge to others.

To say a 'pious' person has the knowledge of unseen, or like Allah knows about everything, or they are aware of all our affairs or they can tell what is happening far and near.

Shirk fil Sama' and Basr, Attributing Allah's power of Seeing and Hearing others.

To say a 'pious' person could hear things far and near or could see all of our acts.

Shirk fil Hukm, Accepting any other person as superior and obey him like Allah

Say a 'pious' person has told one to say certain prayers befor Asar, and the person does it and even delays the Asar prayers and makes it Makruh, it will be Shirk.

Shirk fil 'Ibadat, accepting any other person as worthu of worship like Allah.

To do Sajda or bow before a grave or a 'pious' person, or doing Ruku' likewise or to keep a fast in the name of a prophet, Wali or Imam, or to give Nazar or to promise an offering like that etc., or to go round a house line one gouges around Kabah (Tawaf).

Shirk misc.

To ask about heavenly secrets from astrologers

To show hands to a palmist to know about the future.

To offers sacrifice sweets, flowers etc., on graves

To put pictures and pay respects to them

To call any 'pious' person one's rescuer or saviors

To grow hair in the name of a Wali

To turn Fakir in Muharram in the name of an Imam.