let's go along the chain :
DAC : it converts the raw digital signal extracted from a cd to an analog signal, usable by the analog components following (preamp, amp, speakers). Usually, it outputs a fixed and weak signal of 2V.
Preamp : it's the control unit of a system. Basically, it's a volume control and an inputs/outputs selector. In some case, it also incorporates a phono preamp (the signal coming from a turntable is very weak and need further amplification before being usable; it's also in need of some tonal adjustments). Finally, a preamp can be active or passive : a passive preamp is just a volume control and a few switches, it just attenuates the signal. The signal coming out of it can only be 1 or under. An active preamp puts the volume control between gain stages, in order to present a stronger signal to the amplifier and ease the pressure on the output stage of the dac and input stage of the amplifier. It also cures some impedance mismatches.
Amplifier : it's what gives actually power to your speakers. The weak signal of the DAC isn't sufficient to move the magnets of your speakers (it's in the order of a fraction of a W). The amplifier scales up the signal.
An integrated amplifier is a box with both a preamp and an amplifier.
A receiver was initially an integrated amplifier combined with a tuner. Today, it is mostly used for an integrated amplifier combined with a digital surround processor, used for home theater application. They often have more than 2 channels of amplification.