Each row shows a device with hashrate, power draw, efficiency, revenue and electricity cost per day, the break-even electricity price and the daily result. The break-even electricity price is the threshold up to which a device is calculated to be in profit under current network data - it makes devices comparable across coins. Clicking a row opens the device in the calculator for a detailed analysis.
Efficiency in joules per terahash determines how long a device holds up in the competition: as the network hashrate rises, the break-even electricity price rises first for the devices with the highest consumption per unit of computing power. Newer generations are more efficient but cost more to buy - inspected used devices are listed in the same table and can be compared here directly.
The ranking also shows which devices are currently available in the shop. For a detailed calculation with scenarios, payback period and your own assumptions, open the device in the mining calculator; network trends are shown on the network hashrate page.