G stands for Giga, or billion.
T stands for Tera, or trillion.
eV stands for “electron-volt”, and it is a measure of energy.
- If an electron moves from one terminal of a nine-volt battery to the other, its motion energy will increase by nine electron-volts.
To get a sense of scale: The mass energy of a proton, m_proton times c-squared (c being the speed of light), is 0.938 GeV. So if you took a proton and its anti-particle, an anti-proton, and allowed them to slowly meet and annihilate, they might sometimes turn into two photons (particles of light) and each of the two photons would have 0.938 GeV of energy.
An electron has 0.000511 GeV of mass energy.
A top quark, the heaviest elementary particle known so far, has about 172 GeV of mass energy. That’s just a bit below the mass energy of an atom of gold.
The proton-antiproton collisions at the Tevatron are at 1.96 TeV, or 1960 GeV. The proton-proton collisions at the LHC are currently (July 2011) at 7 TeV, or 7000 GeV. The design of the LHC should allow for collision energies twice that high, 14 TeV, after the design flaws in the magnet-protection system found in 2008 are corrected in 2013.