Some more details on amps/receivers.
Let's talk about analog signals to start (the ones that are transmitted over the red/white RCA jacks) because your speakers take an analog signal.
Components like CD players and DVD players generate a very low level electrical signal that contains the audio information. That electrical signal isn't powerful enough to generate audible sound from your speakers, so it needs to be amplified.
In comes the amplifier. An amp simply takes the weak electrical signal and increases the power so that there's enough power to cause the cones in your speakers to move enough for you to hear them. How much the signal is amplified is controlled by a volume (or "gain") control. So, when you turn the volume knob, you are controlling how much amplification is being applied to the original signal.
A pre-amplifier allows you to connect multiple signals to an amplifier, such as connecting a CD player, a DVD player, a radio tuner, and a TV. A pre-amplifier will let you select which source you want to listen to. These days, pre-amplifiers also often contain digital-to-analog converters to allow you to connect digital audio signals (like an optical cable or HDMI), and it will then convert that digital signal to an analog one that can then be amplified.
A receiver is typically a single box that contains a pre-amplifier, one or more amplifiers (for multiple sets of speakers), and a radio tuner.