It's the tonality of what your playing. A tonality is the impression of the music your playing, like sad, happy, angry, whatever. Tonality applies to all scales, and is mostly seen in exotic scales. Like a japanesse scale, well it's called the japanesse scale because it sounds japanesse. You can classify all scales by this, but not all scales are clear like that one (i.e. modes).

Going back to what you said, "how do I know if I'm playing in G major or E minor?". Try playing each as a chord, play a G major chord and then a E minor chord. Clearly you can tell one sounds minor and the other sounds major. This is how you can tell, very simple. You want to play something that is more minor in tone, then you would be playing in E minor. Looking for something alittle more happy (however you choose to classify major), you would be playing in G major.