http://www.coindesk.com/new-service-finds-optimum-bitcoin-transaction-fee/"Network competition
As the number of bitcoin transactions rise, competition for space in each block is heating up. Miners prioritise transactions with the highest fees, working down the list until the block reaches its limit, 750,000 bytes.
Transactions that don't make the cut remain in the miner's 'memory pool', a kind of bitcoin limbo. They may be included in future blocks depending on their priority or fee.
Currently, you can opt out of the fee altogether. However, there has been debate as to whether this should be raised, with a recent pull request to make a 10,000 satoshi minimum to reduce spam on the network.
CoinTape indicates that avoiding a fee is more likely to result in delays to your payment. It could take up to six blocks, or around one hour (blocks are created roughly every 10 minutes)."