Usually, you are told the day of the interview whether your visa will be issued or not. In other cases, you will get a letter of denial from the consulate after a certain processing time. Sometimes further documentation will be demanded.
No reasons have to be stated for a visa denial. However, a denial can be caused by a variety of reasons, including the suspicion that you wish to immigrate into the US, work illegally in the US, or that you have failed to submit all necessary information.
Once your application has been denied, it can, in many cases, take several months or even years before you can reapply for a new visa. While in theory no official waiting period exists before an applicant can try again, experience has shown that there is no point in reapplying or objecting before conditions (e.g. employment, financial situation etc.) have not changed significantly.