What is the difference between a SIMM and a DIMM?

A SIMM (Single In-Line Memory Module) has a single line of connectors. Connectors on each side of the Board are the same. SIMMs are now obsolete.

A DIMM (Dual In-Line Memory Module) has 2 lines of connectors. Connectors on each side of the board are not connected. DIMMs and SoDIMMs are supported in DDR3, DDR4 and DDR5