Countless of times I've heard and read that RAM-memory can have different speeds - denoted as MHz (e.g. 1066 MHz). However, what this frequency really is has never been explained to me and I'm having trouble finding an answer. My best guess is that - since frequency basically means "how many times per second" - the MHz means how many times per second the RAM can communicate with the CPU. Please do correct me if I am wrong. Also: how can you put this in a relationship to the size of the data being processed per second? E.g. how much data in mega-/kilobytes are sent to the CPU from the RAM per second in a scenario where its being pushed to the limit?
Yes, it's the maximum number of clock cycles per second that the RAM operates on. With Double Data Rate (DDR) RAM, it actually communicates twice per cycle. So for DDR:
200 MHz clock rate × 2 (for DDR, 1 for SDR) × 8 Bytes = 3,200 MB/s bandwidth
This is why chips are now named for their bandwidth, not their frequency alone. Above chip module is called PC-3200, not 200 Mhz. It's still necessary to know the clock rate, to ensure that the motherboard/CPU can operate at that clock.
See the Wikipedia article on DDR SDRAM for more information.