What it is
Greed is the desire for material wealth or gain, ignoring the realm of the spiritual. It is also called Avarice or Covetousness.
Why you do it
You live in possibly the most pampered, consumerist society since the Roman Empire.
Your punishment in Hell will be
You'll be boiled alive in oil. Bear in mind that it's the finest, most luxurious boiling oil that money can buy, but it's still boiling.
In the Summa Theologiae, Medieval theologian Thomas Aquinas said Greed: “it is a sin directly against one’s neighbor, since one man cannot over-abound in external riches, without another man lacking them… it is a sin against God, just as all mortal sins, inasmuch as man contemns things eternal for the sake of temporal things.” (2, 118, ad 1)