When asking for something I seem to hear sentences end in both ください (kudasai) and お願いします (onegaishimasu). Is there a difference and how do I know when to use which?
Here there's a nice explanation, but I'll quote it here for easy reference, with some additional info:
ください and お願いします are both used when making a request.
The main difference is that onegaishimasu assume some action/favor by the other person. It's also a meaning of "I trust this to you".
ください Kudasai (and the more familiar chodai ちょうだい) it's used when you did a request you are entitled to do. You want something or you want someone of same/lower status to do something for you (verb-te+kudasai).
To add to the answers, it's also a directional thing.
下さい is 尊敬語 for くれる, you are asking someone to do something in an honorific way. This is oriented to be polite toward the person you are asking to do something.
お願いします is 謙譲語 for 願う, you are humbly making a request for yourself. This is oriented to be humble about the request you are making.
"More polite" is determined entirely by the situation. This article gives some crazy examples of how mucked up this can all get. If you are working in a shop, using ください is more polite than お～します, and whatever you do don't use 謙譲語 to refer to a customer's actions (that's just bad form).
If someone offers to do something for you, using お願いします to accept is the proper response. Otherwise they are (generally) interchangeable for day-to-day life.
I just asked my sensei only this evening at my Japanese converstaion class. He explain me that ください is less formal and used with '-て' verbs, but お願いします implies favour involved (and is more formal).
I am not good at english just think about casually yourself
Japanese people are called manners important virtue . It expresses in words . i think you knows, two expressions of differences to the through next view
==== VIEW ====
WHEN USING kudasai CASE, (when ordering your friends; a close acquaintanceship )
SIMILAR expression in enligsh : Water please
Onegaisimasu case, (when ordering not your friends and the others; stranger or one's elder)
SIMILAR expression in enligsh : Would you Give me a cup of water please.
I think that two case of differences is politeness level are different.