When you cut wood, assuming it hasn't been dead or taken down by someother force, it's considered green. Green wood has alot of moisture in it. If you can burn it, it won't produce as much heat as it should.
When you season wood, you salt it first then add pepper...
Just kidding. You actually dry it for at least six months. It reduces the amount of water in the wood. I believe wood must have a moisture content of below 20 percent to be considered seasoned. It takes different amounts of time to dry different woods. Also, you need to split the wood, so that it has more surface area to dry.
Thats one of the drawbacks to firewood. You have to let it sit around for awhile before you can sell it.