I ran a
sudo apt-get upgrade and it upgraded the packages ,
ubuntu-software. Out of curiosity, I typed both into the Terminal and I haven't found a difference between the two. Is there a difference, is yes, what is it? If not, why are there two packages.
Canonical, the makers of Ubuntu, created the Ubuntu Software Center (
software-center) as a user-friendly way to discover and install apt packages (friendlier than Synaptic, which was the package manager at the time).
Then the GNOME project thought, "hey, that's a really good idea. Let's make a software center for GNOME. It'll follow our own user-interface principles, and it'll work on all sorts of different Linux distributions, not just Ubuntu." Over time, they created the GNOME Software Center (i.e.
gnome-software). (Now that I think of it, it's possible the Gnome Software Center already existed first, but Canonical rejected it for whatever reasons, deciding to write its own version instead. I think they were criticized over that decision, but it's their right.)
Both Software Centers existed for a while, but then Canonical changed their minds and decided it was silly to have two teams working on such similar projects. So in the latest Ubuntu release they have totally discontinued the Ubuntu Software Center in favor of using the GNOME Software Center. This will remain the default software center in the forseeable future of Ubuntu releases, and Canonical will work to improve the code and integrate it with Ubuntu.
However, many users who upgraded their installations to 16.04 from an older version (rather than doing a fresh install) now have both software centers, since the upgrader program tends not to like removing packages unless they're not being used anymore.
(disclaimer: I could have the timeline of events wrong; feel free to correct me)