You can read John Hull's book which has a chapter about those Greeks ( delta, .... )
These Greeks are usually used (by option trades ) to hedge the option risks.
Here I can give you a direct answer or tutoring:
Option value is a (mathematical) function of these variables: underlying stock price, volatility and interest rate and time to maturity.
Delta is partial differential of option value with respect to stock price and gamma is the second derivative.
Vega is partial differential of option with respect to volatility.
Rho is with interest rate and theta is with time to maturity.
Those ratios are used to measure how much option value will change if stock price or volatility change. So that is why sometime these Greeks are called risks because they can tell you roughly how much of option value you may lose due change of variables.
Hope these give you a quick start.