Keypresses in X have 8 modifier bits:
Any key can be bound to any of these modifiers, but (as you might imagine) typically the left/right Shift keys are bound to Shift, Caps Lock is bound to Lock, and left/right Ctrl are bound to Control. Conspicuously missing from the list is Alt, which is usually bound to Mod1. Tiling window managers also frequently refer to a single "mod key", which is the modifier key chosen for all the WM's keybindings. For example, if the mod key is Alt, then Alt+J will generally advance to the next window.
Meta, Super, and Hyper are all keys from the old Symbolics keyboard that X still has internal keycodes for (so they can still be bound to modifiers). Super is still around on modern keyboards, as the Win/Tux/whatever key next to Alt. It's usually bound to Mod4. Meta and Hyper are both gone. Alt is sometimes called "Meta" because it replaced Meta's functionality; I suspect emacs is mostly at fault for using Alt and Esc as replacements for meta in its keybindings. However, some people refer to the Win key as "Meta" as well, so you should probably just avoid the term altogether.
Right click isn't a key.