The Oklahoma City bombing incident is one example. You must posses a valid Amateur Radio Operators license of the appropriate class. Other places, Dallas County for instance, do not allow this. There are other regulations but these two are the main ones. Always listen to what net control is saying.
Net control is the ham who is in charge of running the net.
In other words, net control is the boss. It's always a good practice to listen for a few moments before you key up anyway. Make your reports as brief as possible. Don't waste time with big words and such. It is held in January, usually the last Saturday.