It you haven't yet, you may wish to read about the First Noble Truth first.
The second of the Four Noble Truths is the Truth of the Origin of Suffering. It, too, is often misunderstood. This is how the Buddha put it:
What, now, is the Noble Truth of the Origin of Suffering? It is craving, which gives rise to fresh rebirth, and, bound up with pleasure and lust, now here, now there, finds ever-fresh delight.
There is the 'Sensual Craving', the 'Craving for (Eternal) Existence', the 'Craving for Self-Annihilation'.
But where does this craving arise and take root? Wherever in the world there are delightful and pleasurable things, there this
craving arises and takes root. Eye, ear, nose, tongue, body, and mind, are delightful and pleasurable: there this craving arises
and takes root.
Visual objects, sounds, smells tastes, bodily impressions, and mind objects, are delightful and pleasurable: there this craving
arises and takes root.
Consciousness, sense impression, feeling born of sense impression, perception, will, craving, thinking, and reflecting, are
delightful and pleasurable: there this craving arises and takes root.
This is called the Noble Truth of the Origin of Suffering.
(Digha Nikaya, 22. Translated by Nyanatiloka with permission to reprint as long as it is free.)
Please note it is not the same as the common belief that the Second Noble Truth simply states Desire is the cause of suffering.
First off, desire is not the best English translation. The Pali word used in the sutra is tanha. Literally, it means thirst.
But, the Buddha used it to mean craving, or attachment. Perhaps the easiest way to understanding of the meaning of tanha is by thinking of its opposite, for which there is a good English expression, namely letting go.
We are experiencing tanha whenever we just refuse to let go.
Specifically, the Buddha is talking about three types of craving which give rise to suffering, the type described by the First Noble Truth:
The first type of craving is kama tanha, or "sensual craving". Sensual in this context is not the same as sexual, though the word is often used that way.
Rather, it is the craving for and attachment to whatever we can generally perceive through our senses. In other words, an attachment to and craving for whatever is not me.
For example, I have the best computer in the world and let no one else touch it. Worse yet, I don't have the best and the latest, and I cannot stand that my friend does, so I just must get it even though what I have is still performing fine for all my needs.
This type of craving is typically expressed with the words I want, as opposed to I need.
Indeed, the distinction between wants and needs is essential to understanding the Second Noble Truth. We all have "desires" (hence my dislike for the use of the word desire as a translation for tanha) based on our needs: When we are hungry, we desire to eat; when we are thirsty we desire to drink. Even the Buddha after his awakening still had to eat, drink, sleep, etc. Yet, there no longer was any tanha in him.
That is why, for example, fasting is not a Buddhist spiritual practice. It is considered foolish to deprive yourself from the things you need. It will not free you from suffering. If anything, it will make you suffer more. Now, overeating and gluttony, on the other hand, is not satisfying one's needs but wants and, hence, is tanha.
The second type of craving is "craving for existence", or bhava tanha. This is the desire for perpetual life. A most obvious example of this type of craving is the desire to get in heaven and live forever. A less obvious example is the desire for fame and glory: I want to write the best OS, so even after I am gone, I continue to live in other people's memory (that is not to say one may not want to do one's best: If a Buddhist were to write a new OS, he would do his best to make it useful and of high quality; I am only talking about the "wrong" motivation for doing the right thing).
Or, I have children because in my descendants I will continue to exist forever. Again, nothing wrong with having children, it is the motivation I am talking about.
And, yes, this type of craving includes the desire for awakening and enlightenment, the desire to be the best, all good things but for the wrong reasons
There is a story about a man who came to a Zen master and asked how long it would take for him to study and achieve enlightenment. The master said five years. The student did not want to wait that long and offered to work harder on his studies. In that case, said the master, it will take ten years.
Finally, the third type is "Craving for Self-Annihilation", vibhava tanha, or, literally, "Craving for Non-Existence". This is expressed, among other things, in the popular notion of living only once, and of there being nothing after we die. In a more subtle way, whenever you feel something like "I wish I were dead", you are experiencing vibhava tanha. When you are bored, depressed, when you become careless or reckless, when you've seen it all and know it all, and there is nothing anyone else can teach you, you are experiencing vibhava tanha.
When you get into drinking, or drugs, when you join a cult or a gang and let others (a leader or peer pressure) make decisions for you, when you're at the end of the rope, when you contemplate suicide (let alone commit one), when you're feeling sorry for yourself, when you feel the world does not understand you, yay, when you feel morons should not have computers, you are experiencing vibhava tanha.
Now, I said the First Noble Truth describes the symptoms. Using the same medical analogy, the Second Noble Truth is akin to diagnosis: Not only can we describe the obvious visible signs of the problems we experience, we now can go down to the root of the problem. Not only do we know what the problem is, we now know why there is a problem.
It is still only an observation: We know what is wrong and why, but we still do not know how to fix it. For that we need the Third and Fourth Noble Truths.