Press release writing is a form of public relations writing. When writing press releases your job is to generate interest and publicity for your clients product, service, event etc. in a "news worthy" manner so that publications will pick it up. It's a combination of techniques that can be tricky, but it's also a highly learnable skill. Press release writers and public relations writers hone the skills needed to garner favorable publicity. Press releases are written following a specific journalistic format, not like regular articles or sales pitches so some specific writing skills are required.
Press releases are a very common means by which companies release news to the public. An effective press release generates buzz and excitement. It is written with the audience in mind, fulfilling a need or want or solving a problem for them. This is also a form of writing where an attention grabbing headline is an absolute necessity. You have only a few seconds to grab and hold the readers attention. If you want your press releases to be picked up, you have to master writing killer headlines.
The Ten Elements of an Effective Press Release
This link covers in detail 10 elements to writing a press release that is effective and most likely to be picked up.
How to Write a Press Release Successfully
This link gives good advice for writing press releases correctly and also links to places to submit your press releases for free.