Some software can be executed by simply copying it to a computer and executing it with no further ado; no installation procedure as such is required. Other programs are supplied in a form not suitable for immediate execution, and require an installation procedure. Installation may include unpacking of files supplied in a compressed form, copying them to suitable locations, tailoring the software to suit the hardware and the user's preferences, providing information about the program to the operating system, and so on. The installer may test for system suitability and available mass storage space.
Because the requisite process varies for each program and each computer, many programs (including operating systems) come with a general-purpose or dedicated installer – a specialized program which automates most of the work required for their installation.
Some software is designed to be installed simply by copying their files to the desired location, and there is no formal installation process. This was once usual for many programs running under MS-DOS, Mac OS, Atari TOS, AmigaOS since early versions of these operating systems and actually it is very common and "de facto" standard in Mac OS X applications and is also used for many Windows applications. There are versions of some operating systems which do not themselves require installation, and can be run directly from a bootable CD, DVD, or USB drive, without affecting other operating systems installed (but not running) on the machine. Examples are AmigaOS 4.0, differentLinux distros, MorphOS AmigaOS clone, or Mac OS 1-9.
Installation usually implies that once installed, the program can be executed again and again, without the need to reinstall before each execution, until the program is uninstalled or the program prevents further execution, for example by coming to the end of its demonstration period.