If you're lucky, the editor will spend a few seconds looking it over. The first thing she'll do is notice the letterhead -- is it from someone important? -- and the format -- is it professional-looking? Then, if you're lucky, she'll read the headline -- is the subject something promising?
Then, if you're really lucky, she'll skim-read the first paragraph. If you have written that lead paragraph effectively, the editor will take an interest -- and you will have cleared the first set of hurdles.
The editor will assign it to a reporter -- "Bob," she'll say, "take a look at this and see what you can do with it." Or maybe, "Sue, give me ten inches on this story." Your press release has now been promoted to a potential story, since a reporter has been asked to use your information and come up with something that might appear in tomorrow's news pages.
"Hey!" I hear you saying. "It's already a story." Nope, it's a press release. It's potentially news, but it's not news until the news media has taken possession and made it its own.