Under the hood, there have been updates to many core packages, including a new 3.19-based kernel, a new glibc, and much more.
One of the larger changes this cycle is a switch from upstart to systemd as the default for managing boot and system service startup.
Ubuntu Desktop has seen incremental improvements, with newer versions of GTK and Qt, updates to major packages like Firefox and LibreOffice, and stability improvements to Unity.
This screenshots Tour Includes installation and applications.
Ubuntu 15.04 (Vivid Vervet) Screenshots Tour