|By Crookesmoor (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons|
Bitcoins are traded at multiple different exchanges often at significantly different prices. At this time, 1 bitcoin is worth $1,043 at Bitstamp, $975 at BTC-E, and $1,108 at MtGox. That is a 14% spread. If you buy at one exchange and transfer to another exchange to sell at a higher price, money can be made. This is called arbitrage.
If you are an experienced trader you should have no problem making money on bitcoins. If you are a novice, be very cautious.
Strategies for making $ on Bitcoin trading/investing:
1. Buy and hold - they are likely to continue to appreciate over time due to a limited supply and increasing demand. They could also get hacked and become worthless. Bitcoin investing is risky!
2. Buy and sell - short term trades can allow talented traders to make money as the market moves
3. Arbitrage - buying low on one exchange and selling high on another