|You really should do some googling because what I'm about to tell you is probably gonna be confusing.|
1st of all, I believe PC = Personal Computer.
Older memory (PC66, PC100, PC133) was named for the frequency that it ran at...66MHz, 100MHz, 133MHz. This type of memory sends data once per clock cycle & became known as SDR-SDRAM (Single Data Rate - Synchronous Dynamic Random Access Memory) after DDR-SDRAM was developed.
DDR (double data rate) memory sends data twice per clock cycle. This is where things will get confusing. DDR memory isn't named for the actual frequency it runs at, it's named for the theoretical double data rate frequency. For example, DDR200 memory actually runs at 100MHz, but since it can theoretically transfer twice as much data at 100MHz as PC100, it's said to run at 200MHz.
DDR memory is also named for the amount of bandwidth it can handle. DDR200 is also know as PC1600 because it has a theoretical max data transfer rate of 1600MB/s. Here's a few more:
DDR266 aka PC2100, runs at 133MHz frequency
DDR333 aka PC2700, runs at 166MHz frequency
DDR400 aka PC3200, runs at 200MHz frequency
The 2nd generation of DDR-SDRAM is called DDR2 & when called by it's bandwidth, it's PC2. For example: DDR2-800 aka PC2-6400, runs at 400MHz frequency.
Then there's the 3rd generation, DDR3 or PC3. For example: DDR3-1600 aka PC3-12800, runs at 800MHz frequency.