A nym account is like a forwarding email address except that it offers
the additional feature of anonymity. Not even the nym server operator
knows who you are! You set up an account with one of the three nym
servers (see #3.2 and #7.2) by sending a config message. In it you
provide a newly created PGP public key for your chosen nym (say,
boozehound@redneck.gacracker.org), some configuration options (like
+signsend, -fingerkey, +nobcc, etc...) and finally a reply block so
the nym server can send any replies back to you through a chain of
remailers of your own choosing, or if you prefer, to a newsgroup like
alt.anonymous.messages or alt.anonymous.

Nyms are different than just sending through anonymous remailers. When
posting through a simple remailer or chain of remailers there is no
way for anyone to reply to your message via e-mail unless you include
a repliable address such as a Hotmail account in the body of the
message, or signature. Additionally, most remailers do not allow any
type of From: header to be posted, so your message will appear to come
from 'Anonymous', 'Anonymous Sender' or similar. When posting through
a nym account, the reply-able nym address remains intact in the
message headers. Reply-able AND anonymous!

Setting up a nym can be done manually (with PGP and a text editor) or
through software like JBN or Private Idaho. Either way you should read
up on the process before trying your hand at it. Here are some very
good tutorials about nym creation:

Nym creation and use for mere mortals
<http://www.stack.nl/~galactus/remailers/nym.html>

Using JBN:
<http://www.skuz.net/potatoware/jbn2/JBNH-en.htm#NymBookCreate>

Also:
<http://mixmaster.shinn.net/nym/index.html>
<http://anon.xg.nu/nym_doc.html>