Tiring things make you feel exhausted or worn out. It's tiring to carry an 80-pound dog through the snow when he refuses to walk.
Tiring things make you tired, either physically or mentally. Digging a hole big enough to plant a tree is tiring, and so is studying for a big math test. You might imagine that the most tiring job would be one in which you do hard physical labor all day—until you spend a day as a preschool teacher with a class of rambunctious three-year-olds! Tiring comes from the verb tire, "to become weary."