Extra Light DME and LME are made from a Pilsner malt as opposed to a regular pale 2-Row malt. Your beer should be lighter than if you used Light extract. The taste might be different, but I'm not familiar with how much flavor difference there is between extra light and light malt extracts. RDWHAHB. Your beer will turn out fine!
Probably not much. There's no formal standard for what counts as Light and what counts as Extra Light, so it depends on the brand that you're talking about. But, that said, it's likely just a few lovibond of color. Unless you're trying to brew the world's lightest pilsner, I suspect the difference will be negligible.