Ăn cơm = Dùng cơm = Eat rice?

Vietnamese people eat rice for lunch and dinner (And a lot of people also eat rice for breakfast, especially they eat "Cơm tấm sườn bì"

Eat rice (steamed rice/boiled rice) = Ăn cơm

When a Western person meet someone, he/she will ask "How are you / How are you doing". Vietnamese people will ask "Ăn cơm chưa?" or "Ăn sáng chưa?"

Ăn cơm is used widely 

But, sometimes you will hear Vietnamese people say "Dùng cơm" 

What is "Dùng cơm"? It is "Eat rice" 

Dùng cơm is honorific form of "Ăn cơm"

If a Vietnamese friend invites you to his house. At lunch time, you should say to his parents: "Mời hai bác dùng cơm" (Well, it is Ok to say: "Mời hai bác ăn cơm)

Hai bác = Parents of your friend

Then, I guess you know how to use Uống nước = Dùng nước = To drink (water)

                                                      Mời hai bác dùng nước