Shirk is an Arabic word which means a partnership. While it is termed in Islamic theology as act of associating partnership with Allah. It is the act of rendering to a creatures respect, honour, obedience or adoration that is only due to the creator (Allah) alone. It is an unforgivable sin which all Muslims should stay away from. Allah said in following verses.
a.) "Verily Allah forgive not that a partner should be ascribed unto Him. He forgives all other (sins) to who He wills. Whose ascribes partners unto Allah has indeed invented a tremendous sin" (Q4: 48)
b.) "......So shan the filth of idol (Worship) and shun lying speech. Turning unto Allah (only), not ascribing partners unto Him, for whose ascribes partner unto Allah, it is as if he had fallen from the sky and the birds had snatched him or thew wind had blown him to a far-off place." (Q22: 30-31).