What exactly is a "virtual core" on Amazon EC2?
The small Standard Instance is:
Small Instance (Default) 1.7 GB of memory, 1 EC2 Compute Unit (1 virtual core with 1 EC2 Compute Unit), 160 GB of local instance storage, 32-bit or 64-bit platform
Does this mean that you get access to an entire physical CPU core? Or are you sharing a more powerful core with other instances?
Is your performance affected by other people sharing the same "physical core" or other hardware?
Answer
You don't get a physical core for a small instance.
"One EC2 Compute Unit provides the equivalent CPU capacity of a 1.0-1.2 GHz 2007 Opteron or 2007 Xeon processor. This is also the equivalent to an early-2006 1.7 GHz Xeon processor referenced in our original documentation." Amazon EC2 Instance Types
You can run cat /proc/cpuinfo to see what hardware you're on.
For example I have a micro instance which has the underlying processor Intel(R) Xeon(R) CPU E5430 @ 2.66GHz.
From my understanding 40% cpu in top equals 1 Compute Unit. So I can burst to 80% with my 2 Compute Unit's.