Casual greeting used among friends or in informal situations. Similar to 'hey' or 'hi' in English.
Hey, long time no see.
Expression of mild surprise, realization, or admiration. Can be used when noticing something unexpected.
Wow, this is amazing.
With a 'heave-ho, heave-ho,' they lifted the heavy box.
やあ is a simple casual greeting like 'hi', while よー can be used the same way but also doubles as an emphatic urging particle.
やあ is a native casual greeting between friends, whereas ハロー carries a foreign, English-flavored casualness.
こんにちは is the standard polite greeting used in most situations, while やあ is a casual, informal way to say hi among friends.
The origin is uncertain; it is a natural exclamation found in many languages, possibly onomatopoeic or a spontaneous vocalization.