Communication involves transferring information from the sender to the receiver. In human communication the sender and receiver are each in separate personal contexts based on their personal history, interests and the reason for which they engage in the act of communication. The data being transferred (i.e. the words spoken or written, pictures drawn, the body language being used, etc) are formed based on the information being processed and filtered through the sender's context and then re-interpreted to information through the context of the receiver. Successful communication relies on the information not being to much obscured through this process.
This said, all human communication occurs between contexts, the recipe for successful communication is that the differences between the contexts are as small as possible. So what is it that makes these context differ? What is it that makes communication hard? I like to use the term "borders". In professional communication, e.g within a project or within a department or a company, many different types of borders can be identified. If you were to communicate without crossing any of those boarders the communication wouldn't be very interesting or valuable, so that is not a success factor. But when laying out your organisation you should take care not to place different borders at the same place since borders tend to reinforce each other and pile up to something higher than the sum of its parts, i.e. making important communication unnecessary hard.
Let's have a look at some commonly found borders in professional life to make the idea clear.
Borders between professions
It is easy for a developer to talk to a fellow developer and for a tester to talk to another tester since they share the same professional context, same professional interests, similar experiences, etc. It is much harder for a developer to talk to a tester or a requirements analyst since that communication is crossing the border of professions.
Borders between area of responsibility
Area of responsibility could be the project you are currently working in or the sub-system (within a larger project) that you are assigned to, but it could also be the requirements for a project as opposed to the code base of the same project. People with different areas of responsibilities tends to have different focuses and priorities and hence communicates out of different contexts.
Borders between geographic locations
Many software projects of today are distributed over several different locations. It might be different countries, different cities, different buildings or as little as different floors or even different rooms. Never the less we, being humans, quickly develops a common context with those people sitting in the same room as opposed to those at another location, hence communication over geographical boarders is harder. Having to consider not being able to speak face to face with people at other locations but being bound to telephones, e-mail, IM, etc, doesn't get communication any better
Borders between chains of command
People separated into teams with different managers also tend to have a harder time communicating than those being assigned to the same team. In some cases management policies might not allow any communication except through the chain of command and that is obviously a problem. But even when direct communication is allowed it suffices from the different context resulting from different priorities in the two teams.
Borders between cultures
The clash of cultures is pretty common in more or less any software project these days. It might be due to organisational mergers, consultants being brought in to work alongside employees or part of the project being outsourced to another company or another country. The more cultural differences the higher the communication border.
So, what to do with all this? It is something to think carefully about when setting up your project organisation. E.g. consider a pretty common organisational pattern: You start with placing, analysts, architects, developers, testers and CM:s each in their separate team with a separate manager only reporting to you, being the top line project manager. Then you decide that development should be outsourced to another company placed in another country (typically with lower cost per developer) and you provide them with no other means of communication than e-mail, IM and phones. Said and done, after some time you start to wonder why there is so much frustration within the other teams, everyone saying it is impossible to work with the development team. Yeah, not so hard to understand, right? You have managed to put the development team in a context where all borders, profession, area of responsibility, location, chain of command and culture, are placed at the exact some place, i.e. right between the development team and the rest of the project. Given this you should be happy if any communication at all is taking place.
Of course, it isn't possible to have the whole project in the exact same context, since it wouldn't be very practical to have only developers or only testers making up the whole project. So, some cross-boarder communication is necessary and that is just find. Usually it works well if you take care to set up your organisation so that borders do not coincide. E.g., this is one of the reasons why cross-functional teams are good. The team members have to cross the professional boarders and perhaps also cultural once to communicate but they are all within the same area of responsibility and (hopefully) in the same location. If you need to have an off-site development team (perhaps with another cultural background) it is a very good idea to ease the communication by having at least one developer on-site handling the communication with the development team over the geographical boarder.
The bottom line advice: Watch out for context borders and don't let them pile up to destroy communication within your project!