PCT applications publish in the same manner as national or domestic patent applications. There is an internet publication of the application on the WIPO website generally on the Thursday that falls 18 months from the priority date of a patent application. The publication includes the front page bibliographic data, abstract, the international search report, specification, claims and drawings. (Note the written opinion is not published at this time).
After publication, the subject matter is prior art as of the publication date and may be able to benefit from provisional protection for published patent applications in some countries (back damages, for example).
Published patent applications can be a great resource for learning what your competitors are doing and keeping up on the state of the art in your technology. To keep a pulse on your field, you can participate in patent watch services or alerts that bring your attention to potentially relevant applications that published recently. (Our office offers such a service for our clients).