Random access memory (RAM) is a computer component that temporarily stores data. A quick tip: memory RAM
for a laptop cannot fit a desktop computer
and vice versa.
SDR SDRAM stands for single data rate synchronous dynamic random access memory. SDR SDRAM and SDRAM are exactly the same terms just the second one is short-term for a first one. This type of RAM is the first generation of this type of RAM and is often used in PCs.
Same as above, just instead of a single data rate, it stands for double data rate. It is double as fast as the above one. It is the latest generation RAM. DDR SDRAM comes DDR2 SDRAM, DDR3 SDRAM, DDR4 SDRAM. If you're looking a memory RAM upgrade, don't mix DDR with DDR2 and so on.