There are numerous definitions of distributed systems. Let's see some definition and then we will try to correlate the aforementioned categories afterward. Coulouris defines a distributed system as a system in which hardware or software components located at networked computers communicate and coordinate their actions only by message passing. On the other hand, Tanenbaum defines the term in several ways:

• A collection of independent computers that appear to the users of the system as a single computer.
• A system that consists of a collection of two or more independent Computers which coordinate their processing through the exchange of synchronous or asynchronous message passing.
• A distributed system is a collection of autonomous computers linked by a network with software designed to produce an integrated computing facility.

Now, based on the preceding definition, distributed systems can be categorized as follows:

• Only hardware and software are distributed:The local distributed system is connected through LAN.
• Users are distributed, but there are computing and hardware resources that are running backend, for example, WWW.
• Both users and hardware/software are distributed: Distributed computing cluster that is connected through WAN. For example, you can get these types of computing facilities while using Amazon AWS, Microsoft Azure, Google Cloud, or Digital Ocean's droplets.