It is not so easy. You can look at the differences here:
http://www.adobe.com/ca/products/pho...omparison.html
Or here:
http://prodesigntools.com/difference...lightroom.html
The biggest thing is how you do stuff. I had Elements first but I wanted more features for Raw editing. Lightroom has the same Raw editing as CS. I also liked the simplicity of LR. In elements the proper way to do something is to open a new layer and apply the changes to that. In LR you do not have to do this (in fact it does not have layers without an add-on like Perfect Layers) you just edit because LR is non-destructive. LR only applies the changes and shows you. Changes can always be rolled back.
Take a look at the links for more detail. The second will have more from a user perspective.