PureOS is the distro installed by default in the Librem laptops. It is a tweaked Trisquel distro with Purism specific changes for their laptops.
Ranked in these QuestionsQuestion Ranking
Pro PureBrowser and TOR
PureBrowser is a modified version of Firefox that comes with security focused plugins. There is also a copy of TOR installed in the OS by default.
Pro You can download and install it, without the need to purchase their laptop
PureOS is available from their site for free for anyone to use, no need to purchase any of their laptops in order to check out the OS which is a tweaked build of the Trisquel distro.
Pro Default install on a free, open source laptop
PureOS is the default OS installed on the Purism Librem laptops.
Con Slower updates
Updates are slower than an unmodified Debian.