Generally speaking an expansion board brings additional capabilities
to a dev kit board. For example in Cypress wireless there is a basic
board with WiFi transceiver on it, and an expansion header. Into that
header they have an expansion board that carries light sensor, thermistor,
Or the power supervisor expansion board for the DVKs CY8CKIT-030 and
Usually the DVK contains the PSoC-device while the expansion-board offers the desired properties as temperature-sensing, fan controlling etc together with some glue logic. The expansion boards are connected to the DVKs through the expansion bus already contained on the DVKs.
In Cypress expansion boards include CAN/LIN transcievers, LCD interface,
CapSense, bare unpopulated proto boards......many varients.
I may not have been exactly clear, the types of capabilities I mentioned
are not on every expansion board, just varients offered as their own board