I am trying to use the su command in Terminal, am asked for my
password, I give it correctly, but I get su: Sorry.

I changed my password and tried the new one with the same response.
Does anyone know why Terminal won't recognize my password and what I
can do about it?

I am running OSX 2.6

Thanks.

In Mac OS X, the root user is disabled by default. When you use the
"su" command with no arguments, you are effectively trying to login as
root; since root is disabled, no password will be accepted.

The root user is enabled in the NetInfo Manager utility, found in
Applications/Utilities. After launching NetInfo Manager, choose
"Authenticate..." from the Security menu, and enter an administrator's
username and password. (The first user created on a Mac OS X
installation has administrator privileges by default.) Then, select
"Enable Root User", also from the Security menu. You will be prompted
to set a root password, which may be the same as or different from
your own password, at your preference.

See:
http://docs.info.apple.com/article.html?artnum=106290

That was very quickly replied to and very helpful. Thank you.