Legal separation and declaration of nullity

Q: What is the difference between legal separation and declaration of nullity?

A: The grounds or reasons are different, and more importantly, in legal separation, the spouses are not allowed to get married again to other parties. The wife also still has to use her husband's surname. (Practically no one therefore wants to file for legal separation; almost everyone in marital troubles will choose to have his or her marriage decalred null and void.)