Difference between back-up computer vs. complete copy of everything?
Time Machine (TM) will allow you to restore your computer to the way it was by copying the the TM files onto the disc in your computer.  When you do a TM restore, you will be back to the same state you were in at the time of the last backup.  There are a few caveats to this.  It assumes that your computer's hardware is fully functional--you have a good display, a good hard drive to copy to, etc.  It also assumes that, for a reasonably large TM backup, you have the time to spare to do the restore.  This can be easily half a day until you are back up and running.

When you create a bootable backup, you may also be able to restore from this backup, but the key advange is that it is immediately bootable.  No need to wait many hours for a restore, you can quickly get working again.  Additionally, you can use any hardware.  Lets assume that your computer is totally fried; just take your bootable backup to another machine, boot from it, and it appears to be your original machine.  Email is configured, documents are available, applications can be run.

My personal backup strategy is to use both TM backup and bootable backup.  If I have the time to restore from TM, I will do that.  I make my living from my Mac, so I have the bootable backup for emergencies.  I can be up and running quickly if I am under a tight deadline.  Hard drives are inexpensive, so my recommendation is to do both!