Portable Apps are software titles that you can take with you wherever you go, without the need to install on each new PC or Laptop that you decide to use.
You can take your favourite browser like Google Chrome or Mozilla Firefox with you and have all of your History, Bookmarks and stored data ready to use.
You could have Libre Office (An MS Office equivilant of Word, Excel, Powerpoint and OneNote) configured with your templates and take it to Work, School or a Friend house and work the same way everywhere.
PortableApps.com compile a list of tested software from lots of major Open Source software developers. You can then install the PortableApps platform on either a removable disk, SD Card or even a cloud account such as SkyDrive or Dropbox. Then when you plug in your USB Flash Drive or open the PortableApps Launcher via a cloud folder you have access to all of your favourite Apps. (Example shown below)
You can then use all your Apps everywhere for Free! without worrying about which settings and customisations you’ve made on each computer, or remebering which PC you book marked that interesting online article on.