what is the difference between civil partnership and marriage?

In the country of Denmark the difference is as follows:

A marriage is entered in a church of an officially recognized religion
in front of a priest. Whatever is said or happens during the ceremony
depends on the religion of the specific church. At the end of the day
the priest will ship off documents to a central registry indicating
that the two parties are now officially married.

A civil partnership (called civil marriage in Denmark) is entered at
the town hall in front of the mayor or an assistant appointed by the
mayor. This procedure has no religious aspect - you are simply asked
to identify yourself, sign some papers, and off you go. Here it is the
town hall that sends off documents to the same central registry
indicating that the two parties are now officially "married".

The civil partnership gives you the same legal rights as a church
marriage - but is not associated with religion at all.

The obove refers only to heterosexual couples.

Gay people are contrary to popular belief not able to marry in Denmark
in front of a priest. They are referred to entering a "registered
partnership". The "registered partnership" is entered in exactly the
same way as a "civil partnership" between heterosexuals - at the town
hall in front of the mayor. The "registered partnership" gives gay
people the exact same rights as straight people - with one exception;
they can not adopt third party children. They can on the other hand
adopt each others children. So for example two lesbians with their own
children can become the legal parents of these children.

I guess a short answer could be: "Marriage is a religious bond between
two people as well as a social contract in the eyes of the law. A
civil partnership is just a social contract in the eyes of the law."