"Technically" speaking: Light side did start out as "good" and dark side "evil" but that's been changed since it's so cliche and it's hard to write anything remotely good.
It's slowly modified in events so Light is "lawful" (lawful good, neutral or evil), and Dark is "chaotic" (chaotic good, neutral or evil.) Basically, all those DnD terms mean this:
Light: "As long as you obey some rule of some sort, consistently" (means affect final outcome)
Dark: "SCREW THE RULES! I have money" (end justifies means)