When an application becomes large, it is usually prudent to divide its source into separate files. This makes the individual parts of the program easier to find and speeds up editing and compiling. When you make a small change to one file, just recompiling that file may be all that is necessary to bring the whole program up to date.
The drawback of this approach is that it is difficult to keep track of many separate files of source code. If you want to load the whole program from scratch, you need to load several files, which is tedious to do manually, as well as prone to error. Similarly, if you wish to recompile the whole program, you must check every file in the program to see if the source file is out of date with respect to the object file, and if so re-compile it.
To make matters more complicated, files often have interdependencies; files containing macros must be loaded before files that use them are compiled. Similarly, compilation of one file may necessitate the compilation of another file even if its object file is not out of date. Furthermore, one application may consist of files of more than one source code language, for example Lisp files and C files. This means that different compilation and loading mechanisms are required.
The LispWorks system tools, and the System Browser in the LispWorks IDE, are designed to take care of these problems, allowing consistent development and maintenance of large programs spread over many files. A system is basically a collection of files that together constitute a program (or a part of a program), plus rules expressing any interdependencies which exist between these files.
You can define a system in your source code using the defsystem macro. Once defined, operations such as loading, compiling and printing can be performed on the system as a whole. The system tools ensure that these operations are carried out completely and consistently, without doing unnecessary work. A system may itself have other systems as members, allowing a program to consist of a hierarchy of systems. Each system is treated independently of the others, and can be used to collect related pieces of code within the overall program. Operations on higher-level systems are invoked recursively on member systems.
It is also possible to define a system in your code using asdf:defsystem, an open source system definition utility with similar functionality to LispWorks defsystem. See Using ASDF for a description of how to use ASDF with LispWorks.
LispWorks User Guide and Reference Manual - 20 Sep 2017