According to System Information I have one processor and 2 cores, but when I open the 'CPU-usage' window in Activity Monitor I see 4 graphs What does this 4 graphs mean and how are they related to my 2 cores?
I have a MacBook Pro (13 inch medio 2012?
You have a multi-threaded CPU. Your computer has one processor, with two physical cores, and two virtual threads on each core. So to the computer it sees four cores/threads available. This makes it easier for load balancing and sharing of the CPU between processes. See this Wikipedia article.