That's the first line of a classic Chinese couplet (duìlián). The second line usually goes: Niūniū qiān niú， niú niù， niūniū níng niú. It translates to "little girl leads along the cow, the cow is stubborn, the little girl pinches the cow".
I had no idea. Even as an ABC who still speaks semi-competently, I'm having trouble figuring out Niūniū, niù, and, níng. What are the characters? Isn't pinch 捏?
妞妞牵牛，牛拗，妞妞拧牛。妞 means girl, 妞妞 is a common name for little girls; 拗 has two different pronunciations, ao and niu; the ao pronunciation is usually used as a verb for bending something, while the niu pronunciation is usually an adjective for stubbornness. I guess 拧 is better translated as to wring or to squeeze and twist, but you get the idea.
má http://mandarin.about.com/library/audio/tones/2.mp3
vs.
"Bring me the mǎ"
mǎ http://mandarin.about.com/library/audio/tones/3.mp3
One will get you some hemp, the other a horse.