A very good question.
What generally happens is that your press release is issued on the 'wire' (actually via internet these days) and news papers, tv stations, radio stations, and even internet news outlets these days then briefly scan all the incoming press releases from the agencies. This mass of press releases will include all things from 'A cat survives nine years stuck in drain', 'New bad breath treatment found', 'New car seat design to be released at Atlanta car show', 'Death of honey bees could cause millions to starve', 'GM auto manufacturers lose $4.2 billion', 'New manager announced of local football club', 'Olympic synchronized swimmer wins $5million dollar damages for scratch on face', 'Domainer selling domain for $75,000 will revolutionize internet', 'Lottery winner leaves $20 million to cat', etc., etc.. There are literally thousands of these a day.
The chances of your press release being read past the headline is quite small, even smaller is it that someone will actually read the whole press release, and of those that do the chances are extremely slim that they will run a story on your press release. So if your press release is not picked up by the news and used (this is what 'picked up means) in effect your money has been wasted.
This may sound brutal, but let me assure you I learnt the hard way. It is generally a waste of money unless you have a very good public interest story behind your press release. I can hand on heart tell you that you stand more chance if you find a reporter who has writen on the subject before and email them your story, especially if you follow it up afterwards by telephoning them and nudging their memory. It will also cost you a lot less. If the reporter does write it up then you are much more likely to have other papers and tv get interested in the story as they avidly read and watch all other newspapers and tv stations for stories so they are not left behind.
Sorry this has been such a long post, but as I say I have learnt the hard way, literally $xx,xxx's of dollars the hard way, and I do not want another domainer to get suckered.
If I can be of any other help please do not hesitate to PM me.