A plasma is an ionized gas where at least one electron has been stripped from a significant number of the atoms in the gas. This makes the gas electrically conductive, as well as, responsive to magnetic fields.

A volt is simply the unit of electrical potential. See here.
The magnetotail is simply the stretched out portion of the Earth's magnetosphere on the side facing away from the Sun. If you imagine the solar wind impinging on the magnetosphere, it tends to compress the side facing the Sun and stretch the side facing away.

__________________
"A mystic is a person who is puzzled before the obvious but who understands the nonexistent." -- Elbert Hubbard