Objective of the game is to guess a word with the help of clues given by number of bulls and cows.

Number of letters which are at the same position as in the word and your guess are shown by number of bulls.
while cows show the number of letters common in the word and your guess but not at the right position.

Example:

If word is 'RATE' and your guess is 'PAIR'

then Bulls: 1 ( for second letter 'A') and
Cows: 1 (for letter R which is present in your guess but at different position in PAIR)

with the help of these clues you have to guess the word.