Please read alien's man page for general documentation.
The original version of alien is still available at the alien home page; drop by http://kitenet.net/~joey/code/alien/
But I have no idea for how much longer.
To use alien, you will need several other programs. Alien is a perl program, and requires perl version 5.004 or greater. If you use slackware, make sure you get perl 5.004, the perl 5.003 in slackware does not work with alien!
To convert packages to or from rpms, you need the Red Hat Package Manager; get it from Red Hat's ftp site. If your distribution (eg, Red Hat) provides a rpm-build package, you will need it as well to generate rpms.
If you want to convert packages into debian packages, you will need the dpkg, dpkg-dev, and debhelper (version 3 or above) packages, which are available on http://packages.debian.org
To convert to/from stampede packages, you will need bzip2.
Attention, Slackware, Red Hat, and Stampede users: Bruce S. Babcock babcock@math.psu.edu has put together an "alien-extra" package of all the extra files you need to use alien on a Red Hat or Slackware system. (Debian systems automatically have all required files.)
The Slackware version is at ftp://ykbsb2.yk.psu.edu/pub/alien/alien-extra.tgz The RedHat version is at ftp://ykbsb2.yk.psu.edu/pub/alien/alien-extra.rpm The Stampede version is at ftp://ykbsb2.yk.psu.edu/pub/alien/alien-extra.slp
Alien is really designed to be used to convert from alien file formats to the packaging format used by the distribution you run it on. Of course, it can also convert from your distribution's native format to alien formats, or from one alien format to another. Do be warned though, that if these latter types of conversions are done, the generated packages may have incorrect dependancy information. This is known to be true if you convert a rpm into a deb on a Red Hat system, for example. Even with alien-extra installed, dpkg will be unable to properly calculate library dependancies for the deb it is creating, and you will get a package without any library dependancies.
I know of one program that acts as a frontend to alien - kpackviewer is a package viewer that can convert between package formats by using alien. Its homepage is at http://www.momentus.com.br/users/hook/kpackviewer.html
Corel also appears to have (or had) something in Corel linux that uses alien.
It is helpful to provide a log of alien --veryverbose reproducing the bug. I may also ask for the package that exposes the problem you saw.