An index draw is when you look at the specific spot you want to shoot, and draw the gun in the most direct, linear, and efficient motion to bring the sights to that spot. Generally, the shooter is not looking for the sights until the sights are intersecting with the visual spot they are looking at.
An L shaped draw is when you draw the gun up to your upper chest, then start presenting it forward toward the target. Often (but not always) on an L shaped draw, the shooter is trying to visually pick up the sights while the gun is moving from the upper chest position out to the target.