all together, altogether

All together means as a group:

            All together they weighed 40 pounds.

            We found the books all together or

            We found all the books together.

Note: All together can only be used if it is possible by rephrasing to separate all and together.

Altogether means entirely:

            The trip was altogether worthwhile.

            Altogether the United States uses 339 billion gallons of water each day.

As a noun, one's birthday suit, presumably all in one piece, uses altogether:

            The Style Committee declined to pose in the altogether (meaning nude: note "the").