Impurities are of two kinds: (1) Major impurities: urine and stool of human beings, semen, wine, flowing blood etc.
(2) Minor impurities: Droppings of haram birds, urine of horse and halal animals etc.
If the major impurity is thin and flowing and gets the clothes or body then it is exempted if it is equal to the area of one rupee coin or less than it i.e. if one offers salah without washing it then salah shall be valid but it is makrooh not to wash it and keep on offering salah. But if it is more than the area of one rupee coin then it is not exempted. And if minor impurity smears the body or clothes then if it is smeared less than one fourth of the smeared part then it is exempted and if it is one fourth of the part where it is smeared or more then it is not exempted. The complete details are available in the books of Fiqh and Fatawa.
Note: Study Bahishti Zewar.
Allah knows Best!
Darul Uloom Deoband