A few Trig questions about radians/input/theory/etc. A beginner.

Okay I've been trying my hardest to get the grasp of Trigonometry down, but I think I have overwhelmed myself. I have a few questions and I am going to try to word them so that they are not complicated and won't clutter up the thread.

1. What exactly is Sine (or Cosine)? The reason I ask, is because I was taught two things. One, that Sine is the Y-value on the Unit Circle or Graph. Secondly, that Sine is the ratio of Opposite/Hypotenuse in a Right Triangle. This confuses me because one seems to be a ratio and another a value (in Radians). I'm sure I've confused myself over nothing.

2. When I enter a value, such as X = -500 for Sine (y = Sine x), what does this value actually mean? Does it mean -500 degrees? Or -500 Radians? On a calculator, I plugged in -500 sine (in degrees) and it gives me -.64. -500 sine (in Radians) gives me .46. What do those values correspond to and why are the signs different? I assume they are the point on the Y-axis, but why are they different values and signs?

I guess I will stop there, since the answers to these may answer my other questions. Thanks to anyone that helps me!