Why is the CPU usage reported by top in Linux over 100%?

AFAIK top shows the CPU usage in relation to the power of 1 core, i.e if your task takes up 200% CPU then it's occupying two cores.

From the top man-page

   k: %CPU  --  CPU usage
  The task's share of the elapsed  CPU  time  since  the  last  screen
  update,  expressed as a percentage of total CPU time.  In a true SMP
  environment, if 'Irix mode' is Off, top will operate  in  number  of
  CPUs.  You toggle 'Irix/Solaris' modes with the 'I' interactive com-
  mand.

So, yes, top shows CPU percentage PER core, but you can toggle this behaviour be pressing 'I'.

Yes, if a process in top goes over 100% or top shows >1.0 in load, this means that more than one core is occupied, or the system is oversubscribed. Oversubscription means that there is more work to be put onto the core than is phsyically manageble.

In top you can also sort the view by different parameters (like used memory). If you want to look what you can sort by, refer to the help or man page for top.

You can also see all kind of information about the processes themselves, like Virtual or resident memory used by the process and if it's sleeping or currently running, as well as cpu time used.

The load average in the top, if needed, can also be viewed by the uptime command.