BU means "Brilliant Uncirculated". This means that the coin has never been exposed to the wear and tear of circulation. It means that the coin has no wear whatsoever, though it may have some contact marks from other uncirculated coins in the course of post-manufacture processing at the mint.

A Brilliant Uncirculated (BU) coin is also distinguished from a Proof (PR) coin, which is a coin made under stricter controls for collectors rather than everyday circulation. Proof coins typically have mirrored surfaces, and are exceptionally well struck from slow moving hydraulic presses. BU coins, also known as "business strikes" are made on high speed electrical presses.

As to what form of gold coins to collect or invest in, I recommend pre-1933 US gold coins in the best condition you can afford.

For more on gold coin collecting, including some spectacular photos to inspire you, see my website at http://www.valuable-coin-stories.com/us-gold-coin.html