What is the difference between transmission and transmission-daemon?

Take a look at the package descriptions using apt show <name>. They usually tell you exactly what the package consists of.

• The transmission package installs a regular client app (either the graphical GTK or Qt5 app, or the command-line one – no idea how 'apt' prioritizes them).

  • This is actually just an empty "metapackage" which only causes more packages to be installed through dependencies. In reality, each of the three versions has its own package: transmission-gtk, -qt, -cli.

• The transmission-gtk and transmission-qt packages install GUI apps meant to be used on a graphical desktop. (Though they do support remote access too.)

• The transmission-daemon package installs a service which always runs in background and is exclusively meant for remote access, either through the web UI or using a "remote" client.

• The transmission-cli package installs a command-line app which can only download one torrent at a time. (It is not a real terminal-based equivalent of transmission-gtk/qt, unfortunately.)

You can have all these packages installed at once, as long as you don't try to run the apps at the same time (GTK and Qt versions share all settings and torrents, so running them both at once would result in a mess).