What is it?
Portable GIS is a windows-only set of open source GIS packages ready to install or copy onto a USB stick, which can be run with no additional configuration. It loads into your existing environment without needing to reboot.
What is it not?
It’s not OSGeo-Live or OSGeo4W
OSGeo-Live provides you with an entire open source GIS operating system, but one that you have to boot into, so it’s separate from your day to day environment. Portable GIS provides a limited subset of the same tools, but accessible in your own environment.
OSGeo4W provides a really easy way of installing all the open source GIS tools on windows, without configuration headaches. Portable GIS provides a subset of those tools, but without the need for installation.