1. What's the difference between a crystal malt and base malt?
Base malt is the "base" of your beer (typically two row pale malt for most ales for example) and will be used in the highest percentage of your grain bill. It will (obviously) contribute the bulk of the fermentable sugar to your batch.
Crystal malt is a roasted malt (some more than others) that will take up at most 10% or so of your grain bill and it will not be a significantly contribute to your fermentable sugars. Crystal malts add flavor, color and body to your finished beer.
2. When looking at steeping grains why do I see 10L and 20L what's that refer too? My guess is color.
The "L" is the lovibond rating of the crystal malt. The higher the number, the longer the roast and darker the color.
3. When putting one together how does one determine the SG to target a certain ABV? (Without using any beer software)+
A very, very rough estimate: If you're wanting a 5% beer, you'd want to shoot for an original gravity of 1.050, if you want a 6% beer, shoot for an OG of 1.060 etc... Your attenuation will determine the final ABV, but these examples should land you pretty close.