Hi there,
Welcome to SimV. I apologize if I don't completely understand your question, but ill do the best to answer your questions. If your referring to Virtual Airlines and Flying clubs, they usually state if whether they fly online or not. Some just ask that you to file a report of each flight you did offline (called a PIREP and usually a form on their website)... some ask you to contribute to online ATC sessions. In any case, the best thing to do is to ask them.
If your just referring to online flight itself, theres a number of places you can goto. First is zone.com. Second is a program called FSHostSpy that will help locate online flight sessions that you can fly in. http://www.chocolatesoftware.com/fshostspy/ If you want to get into the really complex stuff and have real live ATC in your flights, id suggest
http://www.eskyworld.com/ or
http://www.vatsim.com/I like to fly online personally, everyonce in a while, I will stop by on wspilots's server on FSHost. Sometimes I like to fly with ATC and I goto esky.
Anyways, I hope this remotely answer some of the questions you are wondering.