One of the most important features of the Wolfram Language is that it is an extensible system. There is a certain amount of mathematical and other functionality that is built into the Wolfram Language. But by using the Wolfram Language, it is always possible to add more functionality.
For many kinds of calculations, what is built into the standard version of the Wolfram Language will be quite sufficient. However, if you work in a particular specialized area, you may find that you often need to use certain functions that are not built into the Wolfram Language.
In such cases, you may well be able to find a Wolfram Language package that contains the functions you need. Wolfram Language packages are files written in the Wolfram Language. They consist of collections of Wolfram Language definitions which "teach" the Wolfram Language about particular application areas.
If you want to use functions from a particular package, you must first read the package into the Wolfram Language. The details of how to do this are discussed in "External Programs". There are various conventions that govern the names you should use to refer to packages.
There are a number of subtleties associated with such issues as conflicts between names of functions in different packages. These are discussed in "Contexts and Packages". One point to note, however, is that you should not refer to a function that you will read from a package before actually reading in the package. If you do this by mistake, the Wolfram Language will issue a message warning about the duplicate names and use the one last defined. This means that your version of the function will not be used; it will be the one from the package. You can execute the command Remove["name"] to get rid of the package function.
|Remove["name"]||remove a function that has been introduced in error|
The fact that the Wolfram Language can be extended using packages means that the boundary of exactly what is "part of the Wolfram Language" is quite blurred. As far as usage is concerned, there is actually no difference between functions defined in packages and functions that are fundamentally built into the Wolfram Language.
In fact, a fair number of the functions built into the core Wolfram Language are actually implemented as Wolfram Language packages. However, for most Wolfram System installations, the necessary packages have been preloaded, so that the functions they define are always present.
To blur the boundary of what is part of the Wolfram Language even further, "Automatic Loading of Packages" describes how you can tell the Wolfram Language automatically to load a particular package if you ever try to use a certain function. If you never use that function, then it will not be present. But as soon as you try to use it, its definition will be read in from a Wolfram Language package.
As a practical matter, the functions that should be considered "part of the Wolfram Language" are probably those that are present in all Wolfram System installations. It is these functions that are primarily discussed in this documentation.
Nevertheless, most versions of the Wolfram System come with a standard set of Wolfram Language packages, which contain definitions for many more functions. To use these functions, you must usually read in the necessary packages explicitly.
It is possible to set your Wolfram System up so that particular packages are preloaded, or are automatically loaded when needed. If you do this, then there may be many functions that appear as standard in your version of the Wolfram System, but which are not documented in the Wolfram Language reference pages.
One point that should be mentioned is the relationship between packages and notebooks. Both are stored as files on your computer system, and both can be read into the Wolfram System. However, a notebook is intended to be displayed, typically with a notebook interface, while a package is intended only to be used as Wolfram Language input. Many notebooks in fact contain sections that can be considered as packages, and which contain sequences of definitions intended for input to the Wolfram Language. There are also capabilities that allow packages set up to correspond to notebooks to be maintained automatically.