BDU pants is an abbreviation for Battle Dress Uniform, and these were used by the U.S. Military as standard combat uniform pants from the 1980s until the early 2000s.
TDU pants, Tactical Dress Uniform, are basically the newer version and were designed to replace the ‘old-style’ BDU pants.
Therefore, they are essentially the same, except TDU pants are newer and are the improved version of BDU pants.