Dolby 5.1 is a subset of Dolby Digital, it comprises the use of 5 speakers placed throughout the listeners room,

• A left speaker, to be placed parallel to the television/alternate viewing device, and to the left of it.
• A right speaker, to be placed similarly, but to the right.
• A center speaker, to be placed above the television, or below it, the point being the center speaker should be at the center of the room, to localise dialogue and suchlike.
• A left and right surround speaker, to be placed at the left and right of the listener, NOT behind them.. (That's for Dolby Digital 6.1 EX instead)
• And for those really nice explosions, a subwoofer is essential. Place it whereever it suits you best.

Hook these up to your Dolby Digital decoder/amplifier, and you're set. Put in a copy of 'Saving Private Ryan' into your DVD player and be prepared to be hooked!