You want to buy more RAM for your desktop computer. You find it is frequently memory constrained (e.g., when you look at the Task Manager if it is running Windows or use the top utility from a Linux command prompt. What should you do?
Know the difference between DDR3 and DDR4 RAM. If you want to read an article about the difference between DDR3 and DDR4 RAM, see this link.
Beware, many motherboards support either DDR3 or DDR4. Only a few can support either. Even then the slots themselves are dedicated to either DDR3 or DDR4; one slot can support only one type (DDR3 or DDR4) however.
Aside from the type DDR3 and DDR4, attributes of RAM that should affect the purchasing consideration include the bus speed (measured in Mhz) and the size of the RAM (measured in GB). You want your memory to perform well for the price that you pay.