Kudasai (ください) is a polite word that can be used to request something. It is sometimes written in kanji (下さい).
(Give me) water please = mizu o kudasai (水をください)
(Give me) that please = sore o kudasai (それをください)
In conversational Japanese the particle o is removed.
(Give me) beer please = biiru kudasai (ビールください)
Onegaishimasu (お願いします) is another polite word that can be used to ask for something. In some situations onegaishimasu and kudasai are interchangeable. They are interchange able when you are asking for things or food.
(Give me) pen please = pen o onegaishimasu (ペンをお願いします)
(Give me) bread please = pan onegaishimasu (パンお願いします)
Sometimes kudasai and onegaishimasu are not interchangeable. Situations where you will only use onegaishimasu include; 1. When asking for someone on the phone. 2. When requesting a service.
(Give me) Kim = kimu-san onegaishimasu (キムさんお願いします)
used while asking to speak to someone on phone
(Give me) over seas call = kokusai denwa onegaishimasu
used while on the phone talking to operator