If we could tell the 'd's apart, and the 'o's apart, then we would have 6! = 6*5*4*3*2*1 = 720 different 6 letter sequences. Search out "factorials" for more information.
However, since we cannot tell the repeated letters apart, we must divide by 3! = 6 and 2! = 2 to account for the fact 'd' shows up 3 times and 'o' shows up twice.
We get 720/(3!*2!) = 720/(6*2) = 720/12 = 60 which is the final answer.