The xmodmap program is used to edit and display the keyboard modifier map and keymap table that are used by client applications to convert event keycodes into keysyms. It is a part of the X.Org project.
Keyszer is a smart key remapper for Linux (and X11) written in Python. It's similar to xmodmap but allows far more flexible remappings. Keyszer was forked from xkeysnail which no longer seems actively maintained.
Defines the way keyboards codes are handled in X, and provides access to internal translation tables. It is the basic mechanism that allows using multiple keyboard layouts in X.