Memory footprint refers to the amount of main memory that a program uses or references while running.
This includes all sorts of active memory regions like code, static data sections (both initialized and uninitialized), heap, as well as all the stacks, plus memory required to hold any additional data structures, such as symbol tables, constant tables, debugging structures, open files, etc, that the program ever needs while executing and will be loaded at least once during the entire run.
Larger programs have larger memory footprints. Excessive number of utility classes and methods in a programming language design would increase the footprint for API users even if they did not use non-standard plug-ins or applications. Programs themselves often do not contribute the largest portions to their own memory footprints; rather, structures introduced by the run-time environment take up most of the memory. For example, a C++ compiler inserts vtables, type_info objects and many temporary and anonymous objects that are active during a program's execution. In a Java program the memory footprint is predominantly made up of the runtime environment in form of the Java Virtual Machine (JVM) itself that is loaded indirectly when a Java application launches.
During the 1990s, computer memory became cheaper and programs with larger memory footprints became commonplace. This trend has been mostly due to the widespread use of computer software, from large enterprise-wide applications that consume vast amounts of memory (such as databases), to memory intensive multimedia authoring and editing software. To tackle the ever increasing memory needs, Virtual Memory systems were introduced that divide the available memory into equally-sized portions and loads them from "pages" stored on the hard-disk on an as-and-when required basis.
This approach to support programs with huge-memory-footprints has been quite successful. Most modern operating systems including Microsoft Windows, Apple's Mac OS X, and all versions of Linux and Unix provide Virtual memory systems.
With the proliferation of intelligent consumer electronics or embedded systems, low-memory-footprint programs have regained importance once more. Low-memory-footprint programs are of paramount to running applications on embedded platforms where memory is often constrained to within a few MBs – so much so that developers typically sacrifice efficiency (processing speeds) just to make program footprints small enough to fit into the available RAM. So much so that Sun Microsystems has now brought out a version of its Java Virtual Machine (JVM) for these devices; it goes by the name of KVM. The KVM works on platforms where memory is in Kilobytes as opposed to the Megabytes (or even Gigabytes) of memory available on even a regular home PC.