Not sure if there are any linux gurus here, but oh well. From my experience there are only a few ways in which distros differ.

- The install method. Some use a graphical install, some use a text-only install which is usually harder. Some do automatic partitioning, some make you do it by hand. Some set up grub or LILO for you, some make you do it. Etc. etc. The number of CDs used for the install also differs, along with how much downloading if any is required during install. Installing nowadays is much easier than it used to be though, so it shouldn't be too terrible.

- Package management. Some distros use binaries, and some use source. Binaries = precompiled programs you just download and un-zip, or have a package manager download and unzip for you. Souce = you have to compile everytying yourself, which takes much longer, but can also result in more optimized programs. The distros all have different mangers (RPM, portage, apt-get, etc.) Some require more downloading than others, so be careful. Gentoo requires so much downloading it'd be insane not to have a broadband connection.

- Structure. Some distros put files in different places. The location KDE gets installed to for example varies from distro to distro. /opt/kde, /usr/kde/, etc. There ARE standards, but not all distros follow them. Standards compliance might be something you look into.

- Kernels. The basic linux kernel is distro-independent. But some distros offer patched kernels with other "enhanced" stuff. Gentoo offers tons of kernels in addition to the "vanilla" i.e. unmodified one. So does RH I think.

- Startup scripts. This seems to vary pretty widely between distros. The format of the startup / system scripts is cryptic sometimes. Where the files go, the format of the files, runlevels, screwy crap like that.

- Customized programs. RH for example offers a "customized" version of KDE and Gnome, with RH-specific things added to it (and some stuff taken away). Other distros like Gentoo for example give you straight unmodified standard programs. Gentoo's apache differs from RH's apache, which probably differs from Mandrake's apache, etc.

- Versions. Some distros are more up-to-date than others when it comes to the versions of the packages they offer. Say KDE releases version 3.1.3, up from 3.1.2. The speed with which all the distros pick it up and convert it into their various package-manager formats and release it varies. Some are very very slow and focus on being stable rather than offering the newest version of everything. Some try to get new versions out there as fast as possible.

- Support. Some distros have very good support mailing lists / MBs, others don't. Some try to make you pay for support.

As far as your questions, I don't know what Tomcat or Argo is, but if it runs on one distro it'll likely run on them all. As far as hardware, that's mostly a kernel thing, not a distro thing. But some distros (RH) have some funky hardware-detection programs that can help you set up your hardware. Some offer third-party kernel modules (i.e. "drivers") as packages too. But it's mostly a kernel thing, and it shouldn't matter too much which distro you pick.

If you REALLY want to learn, I recommend http://linuxfromscratch.org/ . You'll learn so much it'll melt your brain. Unless you're an expert you're probably not going to end up with a useable system though. Gentoo is almost as good a learning experience as LFS, but it organizes things for you and really does give you a workable system. I know I'm insanely biased in favor of Gentoo, but oh well. It all depends on how much time you're willing to spend. For info I'd read the distro's install docs, and browse the support forums of the various distros themselves. http://www.justlinux.com/ , formerly LinuxNewbie.org, is also very helpful.