A Linux distribution is a LSB compliant system.
Typically, distributions contain end user applications such as text editors of music players, making it usable out of the box for a large variety of non-programmer users.
This work is left for third party enterprises of community projects such as Ubuntu or Fedora which are maintained by Canonical and Red Had enterprises respectively, and not undertaken directly by kernel developers. Contrast this with BSD systems in which the kernel is maintained together with the basic userland utilities.