Portable code can be run with little to no modification in multiple environments. Portable applications can be run from e.g. a USB drive without modifying a computer's environment.
Portable code can be run with little to no modification in multiple environments. This is typically achieved by either abstraction or specialization.
Abstraction is achieved by using a programming language or library which sits between your code and the platforms it runs on, attempting to obscure the differences between them. Any programming language which does not compile to native code attempts to provide some level of such abstraction.
Specialization requires writing code which is aware of the nuances of various environments and adapts to them. This technique is used by many *nix projects, often assisted by capability-detection tools.
Achieving the best possible portability may well involve using both methods in concert.
Portable applications can be run without modifying a computer's environment, such as by storing all configuration data inside the application folder rather than employing system-level configuration.
Such applications are designed to run without altering files on the computer's hard drive, including system config files, type associations, and the like. The goal of such portability is to ensure the application can run when side effects are undesirable, such as when installed on a removable drive or when a user does not have sufficient permission to install software.
