I know what they are, just wasn't sure on the context you were asking
Stems are actually grouped multi-tracks. So you could have a 'stem' that is the vocal part for a track. But in the original production, that stem may have been made up of many multi-tracks. Layers of doubled vocals, backing vocals, harmonies, separate effect tracks etc. This then gets bounced out together as one 'stem'
Not sure what you mean by if I have any?
I've been sent the stems of tracks that I've remixed by the original artist, that's standard really, then you can take your pick on what parts you want to use in your version.
As for software, are you talking about for DJ'ing or production?