Visa or Passport???

Not to seem stupid, but what is the difference between a Visa and a Passport? We are just beginning our adoption process and will probably adopt from China. However, we are also somewhat interested in Kahzakstan, Vietnam, and Guatemala. We'd like to make sure we get our travel documents taken care of right away so we don't have to worry about it later. Will we need Visas or Passports or both? Thank you. Lucy