cpuinfo is a virtual file located in the /proc filesystem. When accessed, it provides real-time information both about unchanging hardware characteristics of the CPU, and about how it is currently running (most notably, what clock speed it or each of its cores is at).
On systems with multiple CPUs or multiple core CPUs,
cpuinfo contains entries for each.
To access it run:
(Or otherwise open that file.)
For more abbreviated information, you may prefer to run