Why theirs a site that I can't find in googles that I can find in yahoo search?

Tweet

I'm just wondering. Why is their some sites that I can find in yahoo search that I can't find in googles? Does it means googles is blocking some sites so that we can't browse it? Before I only use googles. But one time I open the two search engine and start searching same topic. To my surprise in yahoo I can see sites that googles can't search.