It's more service orchestration. So it's a collection of charms to deploy services in to the cloud. Heroku and Google App Engine are more PaaS Platform as a Service, Juju more designed to deploy to Cloud providers like Amazon, Open Stack, and bare metal.
As for different distros Ubuntu is the only distro that Juju has been tested on, though Juju is all Python so it likely wouldn't take much to port and maintain Juju to other Linux distributions. As for the charms most all are reliant on apt-get so Charms for different distributions that used other package management software, then charms would need to be updated to use those instead.
Like, it helps you set up a Heroku or Google App Engine Application ?
Also, can I use it in another distro/Ubuntu desktop ?