Cow

Cows are a type of NPC found in The Sims 2 for Nintendo DS. They are found in the field outside of Strangetown, near Jebidiah S. Jerky's home. They can be interacted with in three ways; Players can either milk them, or tip them over. They can also give the cow a gift, but only chocolate seems to have an effect. After the Sim has tipped the cow over, it will simply come back up. In the field, there's a red coloured bull. No interactions are allowed with this animal. If a cow is milked after it has been tipped over it will produce a milkshake instead of a glass of milk. However, if the cow has been given chocolate it will produce chocolate milk, or chocolate milkshake (if it has been tipped over). This is only a feature though and is profitable nor more effective in filling the need bar.

Cows also appear in The Sims 2 for PlayStation Portable. They appear in the "Kine Dairy" building in Deadtree, but cannot be interacted with.

Cows and Bulls
Cows are the main animals on the farm but there are some Bulls. Bulls cannot be milked (obviously) and cannot be interacted with. To spot one you need to go to Jebidiah's Home. There you will see Red looking cows and they are the Bulls.

The Sims 3
The premium content Milkin' It Dairy Corral is included in the Farm Fresh Folk Set, and it's to be released on September 5th. SimGuruTaterTot has shared the information about what this cow can do. Cow store content 2 Sims can milk cow to get milk. Milk can fulfill Sim's hunger need. Feeding the cow vegetables will make it produce soy milk, which will also revitalize the Sim's age. I remember Life fruit and ambrosia was a thing when The Sims 3 was released. Now all those benefits can be gained from a mere cow milk. Feeding the cow chocolate (!) will make it produce chocolate milk, which will also boost fun. I don't know where Sims will get chocolate though. We'll find out soon. Sims can turn the milk into cheese. Sims can tip the cow... and make it produce a milkshake Sims can play Tic Tac Hoove to increase Logic skill.