it might have second round in the organisation.
it you see form negative sense:
It is consider to be "No"
This is to be consider as your profile is not clear and put it for second round as it is 50% clear or not clear.
When you don't get proper reply after the first round of the interview, do check with them again for sometimes they will be testing your interest towards the job. Some organisation do this as a part of stress interview too. Am not pretty sure about it.
It can be other way too. may be the answer is no. But don't come to any conclusion without confirming.....
normally these days hiring is a very urgent process and employers do not call the candidates for second or third round interviews. becuase options on the part of the candidates is more now that those days. some employer will say after the first interview "we will let you know" which means you are not selected for the job. instead of saying you are rejected, they very act very diplomat way. another way is calling for second interview to assess the candidate with some other new candidates and compare for final selection.
so it depends and differs from one company to another company. based on the requirement , recruitment and decisions change.
nowadays hiring is a very crucial and fast process. therefore employers will not call the candidates for second time or third time for interview. only few startup companies and traditional companies do so. some employers after the completion of first interview, they simply say, "we will let you know". it means you are rejected from the job. some will call you for second, to compare you with other new candidates. therefore it all depends on requirement of the company. the decision and recruitment will change accordingly.
after interview, if they tell u "we will let you know ", it is not the end.
sometimes the recruiter needs some time to take a decision.
.... has to finish the total interviews.
.... has to compare the results.
hence, it is not wise to come to a conclusion with out comfirmation.