This refers to sex procured through the use of the internet. Going online, communicating with like-minded adult(s) and eventually, having sex. No money is exchanged between the individuals. It doesn't matter if you pay to join the site or just use it for free (you have much more access when you pay).
I have been chatting onine with this chick for months. Well, we finally met and did it almost right away. That was my first net-lay and I am looking forward to more.