Hi Hadi All expressions have the same meaning - the difference is a matter of formality. Hello can be used in e-mails/texts etc to strangers, but 'Hi' is good for friends and colleagues. 'Hey' is the most informal - between friends or family.

"Hello" is the standard greeting. It's neither formal nor informal. It is the best one to use. "Hi" is more informal than "hello." "Hey" is complicated. It usually does not mean "hello," it means "pay attention." For that reason, it could possibly be rude. However, like many things, it can be used as a greeting, it's all in the tone of voice. Some typical contexts: "Hey, excuse me, can you tell me how to get to the bus stop?" "Hey! Shut the screen door, flies are getting in!" "Hey, who left their lunch in the microwave?"

Adding to Helen's answer, "Hey" can also be used to get someone's attention. "Hey you!" "Hey, what is this thing?" "Hello?" can be used to question if someone is listening, can hear you, is paying attention, or even as a sort of general "Why are you speaking to me?" question. "Hello? Did you hear what I said?"