In order to write the most general Wolfram Language programs you will sometimes need to find out global information about the setup under which your program is being run.
Thus, for example, to tell whether your program should be calling functions like NotebookWrite, you need to find out whether the program is being run in a Wolfram Language session that is using the notebook front end. You can do this by testing the global variable $Notebooks.
Determining whether a notebook front end is being used.
The Wolfram System is usually used interactively, but it can also operate in a batch mode—say taking input from a file and writing output to a file. In such a case, a program cannot for example expect to get interactive input from the user.
whether output should be given in batch mode, without labeling, etc.
Variables specifying batch mode operation.
The Wolfram Language kernel is a process that runs under the operating system on your computer. Within the Wolfram Language there are several global variables that allow you to find the characteristics of this process and its environment.
the value of a variable defined by the operating system
Variables associated with the Wolfram Language kernel process.
If you have a variable such as x in a particular Wolfram Language session, you may or may not want that variable to be the same as an x in another Wolfram Language session. In order to make it possible to maintain distinct objects in different sessions, the Wolfram Language supports the variable $SessionID, which uses information such as starting time, process ID and machine ID to try to give a different value for every single Wolfram Language session, whether it is run on the same computer or a different one.
a number set up to be different for every Wolfram Language session
A unique number different for every Wolfram Language session.
The Wolfram Language provides various global variables that allow you to tell which version of the kernel you are running. This is important if you write programs that make use of features that are, say, new in Version 6. You can then check $VersionNumber to find out if these features will be available.
the date, as a DateObject, on which your particular Wolfram System release was created
Variables specifying the version of the Wolfram System used.
The Wolfram System itself is set up to be as independent of the details of the particular computer system on which it is run as possible. However, if you want to access external aspects of your computer system, then you will often need to find out its characteristics.