Whenever I relaunch my terminal application on Mac OS 10.4.6, the shell changes to something different. E.g. I used to work in "sh" and then relaunch, I am suddenly in "bash" mode and none of the most basic commands work anymore. Why can't the unix community agree on one single type of shell?? As far as I know there are also different syntax rules across the various shell types? Why is this? How much easier would live be if there was only one type of shell!! It makes the whole thing also very frustrating for newbees like me...

There are different shells in UNIX for different needs. That being said if you are a noob, I would recomend using the BASH shell. I am not a MAC user but you should be able to set your profile to use a BASH shell or what ever other shell you prefer. Use google or read your man pages for further information on setting up your shell and environment.