Marriage itself has whatever religious and emotional significance it has, and is not recognized by law as separate from Civil Union (although some Civil Unions can be marriage in the traditional sense)
Civil Union is a union of two or more adults, which is legally recognized for the express purpose of creating a stable environment in which to raise children. There are certain legal rights and privilidges which Union provides to those within it, and a Union can also include other agreements. Things like sexual fidelity and certain monetary rights can be negotiated.
Getting out of a Union is a very simple matter if no children are involved. It becomes much harder to do otherwis