The Emission node is used to add Lambertian emission shader. This can for example, be used for material and lamp surface outputs.
Cycles uses a physically correct light falloff by default, whereas Blender Internal uses a smoothed falloff with a Distance parameter. A similar effect can be found by using the Light Falloff node with the Smooth parameter.
Lamp strength for point, spot and area lamps is specified in Watts. This means you typically need higher values than Blender Internal, as you could not use a 1W lamp to light a room; you need something stronger like a 100W lamp.
Sun lamps are specified in Watts/m2, which require much smaller values like 1 W/m2. This can be confusing, but specifying strength in Watts would not have been convenient; the real sun for example has strength 384.6×1024W. Emission shaders on meshes are also in Watts/m2.
Color of the emitted light.
Strength of the emitted light. For point and area lamps, the unit is Watts. For materials, a value of 1.0 will ensure that the object in the image has the exact same color as the Color input, i.e. make it ‘shadeless’.
This node has no properties.
The Emission shader output can both be plugged into the Surface Input as well as the Volume Input of the Material output node.