The baguette is a much simpler shape than the emerald. It is used for smaller stones which don't require the more elaborate faceting of the emerald shape. Baguettes are only occasionally found in larger stones.
A similar comparison can be drawn between the single cut and the more elaborate round brilliant.
My understanding of the difference between baguettes and emerald cuts is that baguettes do not have clipped corners. There may be more to it than that, but I know for certain that is one of the differences.