Shoutbox

portable apps: they quite often are not multi-platform.

In essence any program which can run from its installation directory without the need to access anything else (excl. system devices which are globally available), and which does not depend on user access rights, is portable.

To go a step further, a true portable application also doesn't leave any traces (thus also doesn't leave registry stuff behind).

note: those portable messengers which popup once in a time are certainly NOT portable at all. Eventhough they are advertised as that by their "creators" (note the quotes as all they did was quite often copying everything in the installation directory and putting everything in an home made installer (which quite often even doesn't install the stuff properly) and archived together. Those portable messengers use just the same files, need the same files and leave the same traces as a normal a installation)

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Multi-platform applications: as Adaptus explained, they are written with the lowest common denominator approach.

In some cases this means ASM, in other cases (and which are actually very extremely quite common) Java.

Java is by far the most used and easiest way to create multi-platform apps (for those platforms which has a Java applet of course; which are very many) and it is not that hard at all; it is just a programming language like any other.

Java was designed with the idea to create a multi-platform for designing apps.

A multi-platform app can access platform specific stuff though, the only thing is that for another platform a working alternative is available within the program in a transparent way (aka the user doesn't notice this).

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A multi-platform app is not nessecairly porteable and a porteable app is not nessecairly a multi-platform app.