Delta
The Delta is the ration that compares a change in the price of an underlying asset to the corresponding change in the price of a derivative. This is also sometimes referred to as a hedge ratio as it allows the trader to apply a ratio to calculate the amount of derivative needed to hedge the underlying (often cash based) asset.
The delta of a future, for example, an index future, is usually 1.0. This means that for every point change in the cash index the future's price will move by the same amount. i.e. a point.
The Delta really comes into play with options. An call option with a delta of 0.4 will change by $0.40 for each $1.00 change in the underlying asset. A put option with a delta of -0.4 will change by $0.40 for each -$1.00 change in the underlying asset. Note how the put option's value only takes a positive change if the value of the underlying asset drops. This is the nature of put options. They increase with value when the asset drops. With a put option you have the right to sell the underlying asset at a fixed price.
As an in-the-money option nears expiration, it's delta will move to 1.0 (for call options) and -1.0 (for put options). This is because the "time value" of the option is no longer being discounted because there is very little time value left in the option.