What is a yard?
A yard is 3 feet or 36 inches. In the US you mostly see it on football fields, which run 100 yards between the goal lines, and in the UK short road distances are still signed in yards.
The yard is traditionally the distance from the nose of King Henry I of England to the thumb of his outstretched arm, though its length varied from place to place for centuries. It was only fixed internationally in 1959, at exactly 0.9144 meters.
Nearly every other country uses the meter instead. In the US, bulk soil and mulch are still sold by the cubic yard, which is 27 cubic feet.
What is a decimeter?
A decimeter (dm) is one tenth of a meter, or 10 centimeters. The 'deci' is the SI prefix for 10^-1.
The decimeter is rarely used by itself, since people go from centimeters straight to meters. You mostly meet it as the cubic decimeter, which is exactly one liter.
A decimeter is about 3.937 inches, so just under four. Divide inches by 3.937 – or multiply by 0.254 – to go the other way.