See the page on security for more information.
A: Yes. There is no such thing as a "secure browser." Once your computer has a Trojan Horse program installed on it, nothing on your computer is secure. Evil software could display the "lock" icon but still steal your information. Actual instances of such programs have been found.
See the page on backup for more information.
A: No. Don't trust Time Machine to remember files forever. Also, your backup drive could crash. Burn your tunes to a data DVD before you delete them from your computer.
A: You can tell Time Machine "forget I ever had this file" by right-clicking it and selecting "Delete all backups of WHATEVER." (Thanks, Simon Wagstaff!)
A: Yes. They do different things. iCloud sync backs up some of your files to Apple's network. It will back up mail, address book, calendar, photos in Photos, music in iTunes, and "Mobile Documents" created by Apple's iWork applications. Time Machine backs up all of your files to your own disk.
A: Not legally. Apple says you can install a single purchase of Lion, Mountain Lion, Mavericks, Yosemite, or El Capitan on "all of your computers." Installing on a friend's computer violates Apple's license. (Currently this restriction is not checked.)
A: Yes. Do a full backup of your hard drive first. Then dismount and unplug all external devices before doing an OS upgrade. (This is the extra-cautious approach; sometimes it isn't necessary. Before upgrading, check www.macintouch.com to see if others have had problems.)
A: You are trying to run really ancient software, for computers that aren't made any more. b>PowerPC applications will not work on Lion, Mountain Lion, Mavericks, Yosemite, El Capitan, etc. Upgrade the applications to new versions.
A: No. The PowerPC emulator, called Rosetta, in earlier versions of OS X is hooked into the OS in very deep ways. Apple has chosen to eliminate this feature from its operating systems, starting with Lion (OSX 10.7). Without access to the closed source of OS X and Rosetta, it would be extremely hard for a third party to create such an emulator, and keep it working as OS X evolves. It is possible to create a virtual machine under OS X (using VirtualBox, VMWare, or Parallels) and install a Snow Leopard OS on it, and install Rosetta, and run PowerPC software under that. I have done this with VirtualBox. The Apple end user license only permits running Snow Leopard Server on a virtual machine, and Parallels and VMware will refuse to install from a Snow Leopard client disc.
A: See the iTunes page.