Vega (sometimes known as Omega) is the effect on option prices from changes in Implied Volatility.
Like Gamma, I don't bother to quantify Vega (you can if you have a superhuman mathematical brain). But like Gamma it is necessary to know where vega has the greatest effect.
Vega has the greatest effect when At The Money and diminishes the further you get away from the money.
So what this means is, if you have bought an ATM option at high IV's ( which you want to avoid like the pox, unless as part of a spread), you're praying that IV's don't decrease, because it's gonna hurt. It will hurt less so if your option is away from the money. Conversely if you bought an option at low IV's, you don't care if IV goes up because it's to your benefit.
Example: MEOW is trading at $25 with option IV's @50%, and for some inexplicable reason, you buy the $20 call for $5.50, the $25 call for $2.15, and the $30 call for $0.65
For some equally inexplicable reason IV's fall to 40% during the day, and the price hasn't changed. This is what happens:
$20 call goes from $5.50 to $5.35, you lose 15c
$25 call goes from $2.15 to $1.75, you lose 40c
$30 call goes from $0.60 to $0.35, you lose 25c
So we can see here that vega is greatest ATM.
It's clear why we want to know this, If we're buying options we want to buy at lower end of an options IV' range so that vega will work in our favour ...and of course the opposite if we're writing options.
One more point, vega decreases the nearer we get to expiry.
One more greek to go... Theta