Confusion about the ArduPilot

So I am confused. If i buy the ArduPilot 2.5 Mega, do i need to buy an ArduinoMega 2560 R3 As well?

Also, if I buy the ArduPilot 1 board, can i build it to the 2.5 specs my self?

I guess my real hangup is, what is the ArduPilot actually. It seems like

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