A process defines an instance of a program being executed and its environment. As for files, here we will only mention the most important differences, you will want to refer to the Reference manual for a more in-depth discussion on this subject.
The most important difference is, once again, directly related to the concept of users: each process is executed with the rights of the user who launched it. Therefore, if we get back to the example of the file a_file mentioned above, a process launched by the user darth will be able to open this file in read-only mode, but not in read-write mode, as the rights associated to the file forbid it. Once again, the exception to the rule is root...
You will have guessed from the above discussion that one of the parameters of a process' environment is the UID and GID of the user who has launched it. This allows for the system to know whether what the process is asking for is "legal" or permitted.
One benefit is that GNU/Linux is virtually immune to viruses. In order to operate, viruses need to infect executable files. As a user, you don't have access to vulnerable system files so the risk is greatly reduced. Add to this the fact that viruses are very rare in the Unix world in general. So far there have been only three known viruses for GNU/Linux, and they were completely harmless when launched by a normal user. Only one user can damage a system by activating these viruses, and once again, it is root.
Interestingly enough, anti-virus software does exist for GNU/Linux, but for DOS/ Windows files... The reason for this is that, more and more often, you will see GNU/Linux file servers serving Windows machines with the help of the Samba software package (see chapter Samba in the Reference manual).
Internally, the system identifies processes in a unique way by, once again, a number. This number is called the process ID, or PID. Moreover, all processes can receive signals which is how they can be controlled, but this only extends to the processes that you have launched, not another user's process. The exception to this rule is, again, root. You can stop a process, kill it if it's causing you trouble and so on. In a following chapter, you will learn how to track down a PID and send signals to it. This is useful to terminate and suspend problematic processes.