Handbrake
Handbrake is a video transcoder and DVD ripper that converts many multimedia formats as well as DVDs and Blurays without copy protection into MP4, MKV or WebM container files. Thanks to pre-configured presets, this can be done with one click for numerous target devices (e.g. Apple TV, iPhone, Ipad, Android) or according to quality levels. Even large files with high-res material can be downsized. Handbreak uses a queue to bulk convert even large numbers of files.
Depending on the power of the computer, the transcoding process can take quite some time. To avoid unpleasent surprises, the quality of the result can be evaluated by previewing an excerpt. The H.265 codec has also been supported since version 0.10.
Handbrake builds are also available for Windows and OS-X. The Linux version paints the user interface with GTK. Instead of the graphical version, there is also a variant called handbrake-cli for the command line. But if you are used to the terminal, you might be tempted to install ffmpeg instead, because Handbrake is nothing more and nothing less than a front end for the powerful FFmpeg library.
Installation
Ubuntu ships Handbrake in the community supported Universe repository:
sudo apt install handbrake handbrake-cli
The official PPA for stable releases is gone, there is only a PPA left for git snapshots of the development version. New stable releases are build nowadays as Flatpaks.
Documentation
Handbreak offers online documentation in Englisch and German.
Alternatives
There are many GUIs for FFmpeg, but they are usually worse then Handbrake both in terms of user interface and support.