The value of a bitcoin is tied purely to the laws of supply and demand. In other words, the price is determined only by what the market is willing to pay. If more people want to buy bitcoins, then the price will increase. If more people want to sell, the price will decrease.
Bitcoin’s value can be volatile when compared to more established currencies and commodities. This can be attributed to its relatively small market size, and it means that a smaller amount of money can move the Bitcoin price more significantly. This discrepancy will decrease naturally over time as the currency matures and the market size increases.
For merchants wishing to accept bitcoin now, Coinbase provides a way to avoid volatility altogether by instantly exchanging bitcoin payments to your local currency.