There are many different kinds of affairs and reasons for them. Here are some of the more common ones that I have seen:

The bad relationship affair - when one or both partners are unhappy in the current relationship and seek connection and validation from an outside person.

The sexual addiction affair - when a partner has a problem with sexually compulsive behavior and seeks sexual gratification outside of his or her primary relationship.

The friend turned lover affair - when a partner starts turning more towards a friend for emotional support, and that relationship becomes more intimate than the primary relationship. This may turn sexual or may not.

The fantasy affair (or the Internet affair) - when a partner begins a relationship, especially on the internet, which often starts out as an innocent exchange. But then that relationship starts to look better than the real relationship at home, because it is based on fantasy with two people always presenting their best sides to each other. 

Nothing sends a relationship into a crisis quite like an affair, no matter if it is sexual or emotional, one night or many years. The person betrayed often has a traumatic response which may include:

An affair will tear a relationship down to its foundation. The couple then has the difficult choice of rebuilding or moving on. The couples who decide to rebuild may create an even stronger relationship or they may flounder trying to pick up the pieces.  

If you decide to rebuild, affair recovery is hard work and you don't have to do it alone. Let someone with knowledge and experience help you on your path.

If you decide not to rebuild, the person betrayed may still need some support in overcoming the intense emotion and self-doubt that arises from the affair.