What is the difference between Civil Union and Marriage?

A civil union is a type of union between two people wherein two people will be legally protected according to the laws of the state. Take note that a civil union is not available everywhere. There are some states that will not recognize the civil union, so people would need to be married before they can get the expected benefits.
One benefit that people are supposed to get is federal protection. Some members of the LGBTQ+ sometimes choose to have a civil union so that they can protect their relationship. There are instances when this can be allowed.