Militia existed in the colonies long before the American Revolution.
With the exception of Pennsylvania, colonies required most able-bodied men to
own weapons, to be willing to be called for periodic training, and to defend
their communities from attack, primarily by Indians. This was the colonial
Minutemen came into being in the wake of the crisis in Boston, in 1774.
Essentially, they were a reorganized militia, rid of any vestiges of loyalty
to the crown, and trained, far more extensively than they had been previously,
so that they could "turn out" at a minute's notice.
The Continental Army was an invention of Congress and the command of
George Washington. When Washington arrived outside Boston in the aftermath of
the battle of Bunker's Hill, he immediately began to train the diverse forces
he found there into a regular army. This group became the Continental Army.
The militia continued to exist and fight throughout the revolution with mixed
results. Continental Army officers tended to deride its effectiveness,
probably with reason, at least in the early years of the war. But at Saratoga,
in the South, and in New Jersey during a 1780 campaign, they were essential
fighting forces. By the end of the war, Washington and others in the
Continental command were using the militia as support for the regular army, and
they were a crucial component in the ultimate victory.