Cattle can be bought or stolen from Villages, although stealing them will reduce your Reputation with the village. The price in that village will rise each time you buy cattle, but the cost will reset after some time. The number stolen, and the chance to successfully steal them, is based on your Looting skill. Once you have acquired cattle, they appear on the overland map rather than going into your inventory, and you must 'herd' them, driving them toward the location you want to go. The best method is usually to move behind them in a zig-zag pattern. In With Fire & Sword, the cattle will follow you.
Cattle must be bought from the village elder in the village, you can't buy the cattle directly from the world map.
You may choose to kill some of the cattle. This will net you 2 units of fresh beef for each cow you slaughter.