The tenth commandment forbids all desire to take or to keep unjustly what belongs to others, and also forbids envy at their success.

Take heed and guard yourself from all covetousness, for a man's life does not consist in the abundance of his possessions.  (Luke 12:15)

     The Tenth Commandment According to the Catechism of the Catholic Church

You shall not covet --- Anything that is your neighbor's --- You shall not desire your neighbor's house, his field, or his manservant, or his maidservant or his ox, or his ass, or anything that is your neighbor's.

For where your treasure is, there will your heart be also. (Mt 6:21)

The Tenth Commandment completes the ninth.  While the ninth is concerned with avoiding the sins of the flesh, the tenth forbids coveting the goods of another.

There is nothing wrong with the desire for things.  In fact, it is quite natural.  It's when these desires exceed the limits of reason and drive us to coven unjustly what is not ours, and belongs to another.  This is when we commit serious sins against God's Law.

Greed

The tenth commandment forbids greed and the desire to amass earthly goods without limit.  It forbids avarice arising from a passion for riches; and those riches ability to obtain power.  It forbids you the desire to commit injustices that can lead you to harming your neighbor to obtain his earthly goods.

Just Means

It's not wrong to desire to obtain things that belong to your neighbor; if this is done by just means.  Just Means prevents the struggle to obtain worldly goods by criminal acts.

Envy

The tenth commandment requires us to drive envy from our hearts and minds.  Envy is a capital sin.  St. Augustine called it the diabolical sin.  From envy is born jealousy, hatred and delight in the misfortunes of our neighbor, which when uncontrolled can lead one to do his neighbor great harm.

Riches Can Destroy

The Lord grieves over the rich.  It is harder for a rich man to enter into the kingdom of heaven than a camel to pass through the eye of a needle.

Those who are absorbed in worldly goods and material things are usually hard-hearted for those in need and will never lend a helping hand.  They constantly buy the things they don't need and many are always in financial stress.  This leads to a source of great unhappiness.  (Ask those in credit card debt that can never be paid off)  Desire for the things of this world can lead to total despair.

Summary (According to the Catechism of the Catholic Church)

Where your treasure is, there will your heart be also (Mt 6:21)  If your treasure and heart are with earthly goods your eyes will not see the kingdom of heaven.

The baptized person combats envy through good-will, humility, and abandonment to the providence of God.

Detachment from riches is necessary for entering the kingdom of heaven. "Blessed are the poor in spirit" (Mt 5:3)

Saint Bernadine of Siena