What consists of online infidelity? I know it is a general question... just curious if anyone knows details?

One can not be physically unfaithful online.

But:

If someone spends hours just on a site like this and does not spend time with his family he is cheating them out of their time

If someone is on porn sites they are getting an unrealistic idea of sex and often finds his then physical performances not living up to "online" standards.

If someone is on gambling sites, they may lose needed money for the household

One may check out dating sites and try to find someone to become sexually active with

One may have unknown perversions and find their outlet online.

Computers have the ability to be good or bad, often the abuser does not know it and have to be forced to see.

Think of on-line communication like you would any other conversation between two people.

People may be friendly and flirt, the may even talk about sex, but when their conversation starts going on to explicit sexual acts for each other, I think the line into online infidelity has been crossed.