As synergy said, multisession enables you to burn files to the CD/DVD later on. This works with all CDs/DVDs not only with RWs.
The only difference is that in RWs, you can erase all the data burned on the CD/DVD but on regular CDs/DVS, you can't. You can just add to the existing data.
A multisession disc burned in Nero will only be able to be added on in a different session by Nero.
I've burned CDs and DVDs multisession using Nero and was able to add to them using Roxio.
Similarly, I burned some data using Roxio and was also able to burn multisession using Nero.
(When doing so, a message appears that Multisession was not created by Nero...just click ok and you can continue burning your multisession cd or dvd)
I dont know if it works with all software though