A distribution (often shortened to “distro'”) is a operating system, usually based on Linux (the kernel), the GNU utilities and often other programs as well.
Anyone can make a distribution, and some people do better than others, and some people target different markets or machines. Red Hat, for example, is a multi-million dollar company that produces an operating system named Red Hat Enterprise Linux, aimed squarely at large server installations used by companies needing support contracts, on x86 servers. Yellow Dog is based on RHEL, but recompiles the software to run on the PowerPC architecture (ie. Apple Macintosh hardware).
If you have particular problems then you probably want to treat each one as a separate question and fix it separately. One thread per question works well, since you are unlikely to get answers from one person able to fix all the problems.
Problems with rebooting usually get logged into the file /var/log/messages; try there first and look for error messages that may give you some clue as to what went wrong.
“Id X respawning too many times” means that, for some reason, your graphical display is crashing when it starts; again, the reason for this will probably be in the log file /var/log/XOrg.0.log (or similar).
“startx” will start your X server but it uses different scripts to decide what to run. Without knowing what kind of desktop you are trying to run, and what is actually being run, it could be quite hard to fix. Take a look for a file named .xsession or .xinitrc in your home directory (these will probably be treated as hidden files because the filenames start with a full stop/period).
As krishnasut says, if you are having too many problems then you could try changing your distribution. There are a limited number of distributions that will run on the PowerPC, but Ubuntu is one of them and it has the advantage of running straight off the CD, so you don't need to install it to find out if it will work.
Hope that's of some help, and feel free to post any more questions that you have.
—Robert J Lee