What's the difference between the Union Jack and the Union Flag?

What is commonly known as the Union Jack, the flag of the United Kingdom, is actually only called the Union Jack when it is flown on the jackstaff on the bow of a Royal Naval ship. The correct name for the flag is the Union Flag.
http://www.flaginstitute.org/wp/british-flags/the-union-jack-or-the-union-flag/

It was called the Flag of Britain when introduced in 1606, the Union arriving soon after in 1625 (satguru).

Another place where one might have made the distinction would be the American civil war, where the Confederate flag was known as the Union Jack, as opposed to the Union Flag in the north.

The answer given by Maggs is WRONG! This is a common misconception. The two titles can be used interchangeably and this has been the case since 1902, or officially since 1908.

"The Flag Institute, the vexillological organisation for the United Kingdom, stated that the term Union Flag is a "relatively recent idea". Jack was a word previously used to denote any flag.[4] It also noted that "From early in its life the Admiralty itself frequently referred to the flag as the Union Jack, whatever its use, and in 1902 an Admiralty Circular announced that Their Lordships had decided that either name could be used officially. In 1908 a government minister stated, in response to a Parliamentary question, that "the Union Jack should be regarded as the National flag" ."

http://www.flaginstitute.org/index.php?location=7.2