A more concrete definition of a Distributed System is the following: a distributed system is a collection of computing resources autonomous, interconnected via a network, like Internet World-Wide-Web whose distribution is transparent to the user that sees the system as a single coherent one. The system appears as one local machine functionality, being in a contrast to a specific network, in which the user is aware that there are several machines, having information about them, about their location, and their storages (if access is available)
A distributed system features are:
o Interaction between processes to achieve a common goal
o Processes concurrent execution
o Exchange of information is made through the network
One example of distributed system is itself the World-Wide-Web as it offers the
possibility for diverse number of users to see and access the same information. So one say that the process of seeing information is distributed to many users, without letting them know that maybe at the same time, other users execute the same process.
Other examples of distributed systems are Organizational Intranet, Air traffic Control Systems; the distributed systems are called "the next wave" of computing, as they can be found nowadays in many domains like science, leisure or economy (ex: systems for processes in banks).
The answer to the question of why Distributed Systems are needed is that an application may use resources from more computing points (nodes) or the application is being used by numerous nodes, and it has to be distributed for accessing it.
The problems that a distributed system can confront can be the trade between security domain - (unreliable transfer of messages) and scalability.
