This article lists methods of setting default applications in applications and Linux desktop environments.
Setting the default application via file manager
In many cases it suffices to use your file managers capability to set default applications for specific file types (e.g. vie right click). Please refer to your your file manager's specific manual.
Setting the default application via desktop environment
Bigger desktop environments like GNOME or KDE allow you to set default application by their own means. With the other desktop environments, you can run xfce4-mime-settings from xfce-base/xfce4-settings.
Setting the default application via mimeapps.list files
MIME types describe the kind of content a file contains, which practically determines the applications to be used when opening or editing a file of a specific MIME type. While using a file manager or desktop environment to set default applications may work for the most basic cases (they edit mimeapps.list files for you), the most thorough way to associate file types and applications is by manually editing the MIME configuration files and using dedicated tools.
The location of mimeapps.list files and their precedence is specified by the Freedesktop standard
Setting the default application via xdg-settings
For example, if www-client/lynx is the default web-browser, but you'd like it to become chromium use:
xdg-settings set default-web-browser chromium-browser-chromium.desktop
Looking up the default application of a specified MIME type
To find out the default application for directories use:
xdg-mime query default inode/directory
Looking up MIME types associated with a specific application
To look up MIME types Inkscape is capable of working with use:
grep MimeType /usr/share/applications/inkscape.desktop
Few applications do not honor the Freedesktop standard.