Okay, you have the 2.0 GHz iMac. 2 GHz is the clock speed that the processor runs at. The processor reads the information that is on the hard disk, and opens applications.
You have an Intel Core 2 Duo machine (as do I) this means that it has two processor cores on the same chip. Effectively having 2 processors in one computer. This makes running applications faster than a single core 2 GHz processor, because it can split the work load between the two cores. So some information would go to one core, and the other information would go to the other core.
Santa Rosa is the platform, or type of motherboard that the processor and RAM are built into. The SR platform can handle 4 GB of RAM.
If you need anything explained more, I will be happy to oblige