Auei! is an interjection. It can be used in any moment of surprise, excitement, or just general interest. It can be used in both negative and positive situations.
Must be followed by a phrase that further clarifies your feelings towards the situation.
If you're using it while talking to someone in person, you can make the appropriate motion while exclaiming it. To do this; throw your arms into the air, keeping them stiff and straight, with palms facing upwards. Kind of like making the Y in the Y-M-C-A dance.
Boy: Do you want to go out for coffee?
Girl: Auei! Yes I would love to!
Teacher: You got an D on the math test.
Student: Auei! That sucks!