DAC = Digital to Analog Converter. Sound is heard as waves that are smooth with peaks and valleys...Analog. CD's and most modern sources are digital meaning a bit rate is applied to an analog source (8, 16, 20 or 24) and that represents the number of rectangles under the analog curve used to approximate or rather define how that analog signal is to be stored digitally. The more rectangles or bits you have the more accurate the processing and storage is. If you took Calculus think back to approximating area under a curve. Once the information is processed by whatever bit rate is chosen and stored on a CD, it then needs to be decoded from that CD or DAP back into a smooth analog signal that humans can listen to. Unless you are Neo from The Matrix you probably don't listen to your music in Binary.
Amp = Power Amplifier. Speakers generate sound by moving air and creating vibrations that turn into sound waves. In order for a speaker to reach its full potential to do that it requires a certain amount of power to make those speakers move. Most players will drive a speaker enough to make sound, but usually that sound is not optimal because it is not receiving enough power to make the speaker use its full potential. That's where an amp comes in. Amps can also be used to color or alter a source sound to a desired signature.
For amps, search for Skylabs comprehensive reviews.
For DAC's, I think there isn't much of a centralized source. Search out Burr Brown, Wolfson or Cirrus Logic.
