When turned on, a floodlight projects a beam of light at full brightness until it hits an opaque bloock or reaches a maximum range. The default range is 64 blocks, although this can be increased up to a limit of 255 blocks by changing the maxFloodlightRange configuration setting.
Light from a floodlight has the same properties as torchlight, so it will prevent mob spawning, promote plant growth, and so on. It is not sunlight, however, so it will not ignite skeletons or power solar panels.
Most floodlights require a redstone signal to turn them on. Some types also have an internal switch.