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 native casual greeting between friends, whereas ハロー carries a foreign, English-flavored casualness.
The origin is uncertain; it is a natural exclamation found in many languages, possibly onomatopoeic or a spontaneous vocalization.