1. Amplifier Board
1) Channels - Mono or Stereo? For a stereo setup (powering two speakers) you will need two mono boards or one stereo board.
2) Power - how many Watts per channel (speaker) do you require? See HiFi for Dummies for more information.
3. Volume control (potentiometer) - Some amplifiers will have this, others will not. If not, you will need a separate preamplifier or dedicated volume pot.
3) Sockets - usually screw terminals for all inputs and outputs (Fig 1), sometimes DC socket for DC power input (Fig 2), and sometimes 3 pin connector for audio signal input (Fig 2)
4) Heatsink - some boards will require you to attach a heatsink (Fig 1)
(click photos to expand)
2. Power Supply