Computer processors are really, really fast. Hard drives are really, really slow in comparison. If computers handled and processed data by writing and reading directly from the hard drive, computers would be extremely slow too, because the hard drive couldn't keep up with the processor's speed. RAM is there to address this problem. It's a place where data is temporarly stored that is really really fast to read and write from.
DDR4 ram - ddr4 is the most current dram availble for memory and is currently the fastest with a maximum speed of 4400 MT/s (megatransfers per second).
DDR3 ram - ddr3 is the last generation of ram. This ram is slower reaching only 2133 MT/s.
3200 - Best budget ram
3600 - Best amd budget ram
4000 - Best mid-range ram
4600 - Best high-end ram
5000 - Current gens Fastest