* Wujek (pronounced "voo-yek") is used for your father's brother (the equivalent of "uncle" in English).
* Stryj (pronounced "stree-ee") is used for your mother's brother.
So, you can say:
* "Mój wujek jest Polakiem." (My uncle is Polish.) - if your uncle is your father's brother.
* "Mój stryj jest Polakiem." (My uncle is Polish.) - if your uncle is your mother's brother.