Let's talk about human desire for a minute. Human desire, in and of itself, is not wrong. Even the sexual desire that is a part of humanity is not intrinsically sinful. You see, God created us and He created us with certain desires (e.g., to eat and drink). When you're hungry that is your body desiring to be fed. There is certainly nothing sinful about that desire! But, if you go out and steal from the grocery store in order to satisfy your hunger, then you've sinned. God wants us to work for our food--that is a lawful way in which we can satisfy our hunger. The same could be said about sexual desires. God created them, but He has also established a lawful way in order to fulfill those desires--He calls it the marriage relationship (cf. Heb. 13:4). To go out and have sex before or outside of marriage is wrong. Lust, therefore, in the Biblical sense, is simply unlawful desire, and that is what James is talking about here.
All of the general desires that God has given us have correct ways to be fulfilled or satisfied. But, Satan will try to tempt us to fulfill our desires in ways that God doesn't approve of. In some ways, this reminds me of fishing. Fishing is basically deception--humans deceiving fish. When you throw your baited hook in the water, the fish don't see the hook. They see the big, juicy worm dancing around as you tug the line and try to tease them into biting. If they do bite, did you force them to do so? Of course not. You simply used their own desires against them. They have a desire for food, and that worm has really got them excited! They expect the sweetest satisfaction when they bite down on the bait, but they soon learn that it wasn't what they thought it would be.
Isn't this the same way Satan works on humans? In order for us to sin, Satan will first try to draw us away from safety by our own desires. We must all strive to avoid going to places where Satan could easily hook us. He'll try to deceive us; don't let him do it! Once he's got our attention he'll try to get us to bite into something we shouldn't. We must be strong and not give in--God will provide a way of escape! But, we have to want to escape the temptation or we never will!
Think about it--God, through James, is telling us how temptation works so we can be smart enough not to give in to it! If fish knew about hooks what James tells us about the devil's enticements, there'd probably be a lot fewer fish dinners!
Perhaps all this talk about temptation and unlawful desire has got you wondering about Jesus. Did He have the desire to commit sin? Yes, He did. Hebrews 4:15 states - "For we do not have a High Priest who cannot sympathize with our weaknesses; but was in all points tempted as we are, yet without sin." Jesus was tempted like you and I are. The difference is that He never gave in to those temptations. There is an important lesson here for everyone. Just because you are tempted to do something doesn't necessarily mean that you've sinned. If I'm in the checkout line at Wal-Mart and I see someone drop a $50 bill when they get money out of their purse or wallet, I might be tempted to pick that money up and put it in my pocket. However, just because that thought pops into my head doesn't mean that I've sinned. I need to have the strength to say, "No, that money is not mine; I will return it to its owner."
Likewise, if I'm checking my e-mail and I find a message from some woman I don't even know, and she tries to entice me to click on the link below to go to her web cam, I might be tempted to do it. But, I'm sure it's something I should not be seeing. Now, just because I may be tempted does not mean that I have sinned--at least not yet. In this case, if I give in to my desire and go to the website and fill my mind with its filth, then I have undoubtedly sinned by lusting (Matt. 5:28). Do you see the difference? If I lust after her, then I've sinned, but if I quickly delete her immoral invitation and think on other things, then I haven't sinned.