You buy a contract, for example, to get your next 1000 gallons of gas at $2.20 per gallon. You put down a small deposit, and pay as you go, but you know that the next 1000 gallons will be $2,200, guaranteed.
A speculators role is to back the other side of the contract (to sell it). He is the one guaranteeing your price, so if the average price for the next 1000 gallons is $1.80, you still pay $2,200 in the end, but his cost is $1,800, so he makes $400 on the contract. Now if the price averages $3.30, he pays $3,300. You still pay $2,200, so he gambled and lost $1,100.
Speculators, like most gamblers, will probably bet on almost anything. We need to find more ways for them to take on our risks. Just imagine the many contracts could be invented, based on speculation. Speculators get a bad rap. Speculation in stocks, currecies and commodities futures is a necessary part of our economy. Many people have the idea that there is no added value in people "gambling" on commodities prices, for example. The truth is, most people just don't understand of the role of speculators and speculation.