Use the word cattle to talk about a group of cows. A farmer might build a new fence to keep her cattle more safely secured in their pasture.
Cattle usually refers to domesticated cows, almost always a large group of them. If you have a small dairy farm with only three or four cows, you'll probably call them "cows." When you're talking about a bigger operation, they're more likely to be called cattle. In the 13th century, the word simply meant "property," from the Medieval Latin capitale, "property or stock." It took about 300 years before cattle meant "cows."