How to distinguish DDR3 DDR4 memory module specifications

Error Detection and Correction
Most desktop and laptop computers use RAM chips that hold eight bits per byte, while high-end servers and workstations typically have nine bits. The ninth bit is a parity bit for detecting errors. See ECC memory and RDIMM.

Upgrading Memory - Read the Manual (RTFM!)
A single DIMM can often be used, but pairs of DIMMs increase performance in machines that support dual channel DDR SDRAM. When upgrading memory, read the motherboard manual to find out which module combinations can be used. See MT/sec, SDRAM, memory types, memory card and Hybrid Memory Cube.

DIMM Modules

DIMMs are used in desktop and servers; laptops use SODIMMs.

The notches at the bottom of the modules designate the number of pins.

However, the way chips are placed on the module is up to the manufacturer.

Earlier Modules

RIMM (Rambus) and SIMM modules are no longer used.

RIMM modules transferred data over 16-bit and 32-bit channels,

whereas SIMMs were 32-bit. DIMM modules support 64-bit paths. See RDRAM.

DIMM Modules in a Desktop Machine

To change memory in desktop computers, the cabinet has to be opened.

Two of the three DIMM slots on this Mac motherboard are empty.

Laptop Memory (SODIMMs)

To change laptop RAM, a cover plate on the bottom of the unit typically has to be unscrewed.