Differences in Raspberry Pi Compute Boards

What is the difference between the two Raspberry Pi Compute Boards and what else would I need?

Of the Raspberry Pi Computer boards Digi-Key carries there are few differences between them?

The RASPBERRY PI COMPUTE BOARD// 1690-1003-ND is based on the original Raspberry Pi and Raspberry Pi B+ models), and has a BCM2835 processor with 512MByte LPDDR2 RAM and 4Gbytes eMMC Flash on board

The two other options CM3// 1690-1016-ND and CM3 LITE// 1690-1017-ND are both based on the Raspberry Pi 3 board and have the more advanced BCM2837 processor. The CM3 LITE has No flash memory on board and the CM3 has 4Gb eMMC Flash on board

The pinout of CM1 and CM3 are identical. Apart from the CPU upgrade and increase in RAM the other significant hardware differences to be aware of are that CM3 has grown from 30mm to 31mm in height, the VBAT supply can now draw significantly more power under heavy CPU load, and the HDMI HPD N 1V8 (GPIO46 1V8 on CM1) and EMMC EN N 1V8 (GPIO47 1V8 on CM1) are now driven from an IO expander rather than the processor

What else would I need?

As the compute board system is a standardize SODIMM module you would either need to design your own breakout board or use the I/O BOARD// 1690-1018-ND which can be bought separately or as part of a kit along with the CM3 under part number 1690-1019-ND