Michigan's official flag was adopted by the Legislature in 1911 with a simple description: The State Flag shall be blue charged with the arms of the State (the state coat of arms appears on both sides of the flag). Animal symbols: Moose and Elk represent Michigan, the bald eagle signifies the United States.

Meaning of Latin Phrases on Flag:

E Pluribus Unum: From Many, One
(our nation was made from many states).
Tuebor: I Will Defend
(refers to the frontier position of Michigan).
Si Quaeris Peninsulam Amoenam, Circumspice:
If you seek a pleasant peninsula, look about you.
(also Michigan's state motto).

Michigan's first flag was flown in 1837 (the year Michigan became a state). It featured the the state coat of arms, a soldier, and a lady on one side; and a portrait of Stevens T. Mason (the first governor) on the reverse side. In following years (previous to the official flag being designated) numerous designs were used with the coast of arms, including the United States coat of arms on the reverse.

Official Pledge to Michigan State Flag:

I pledge allegiance to the flag of Michigan,
and to the state for which it stands,
Two beautiful peninsulas united by a bridge of steel,
where equal opportunity and justice to all is our ideal.

Source:

Coat of Arms: Michigan Department of State
Michigan FAQ: Michigan.gov
Getting to Know Michigan: Michigan.gov

Links:

State Flags - 50 states List
National Flag