The SI unit of luminous flux, symbol lm, equal to one candela steradian. (Thanks Gritchka!)
The luminous flux, or lumen dF is given by :

dF = IdR

I being the luminous intensity, or candela, dR describes 'an element of solid angle', the steradian. The lumens tells you the total flux of energy emitted by, say a lamp, over all possible directions. This is different from the candela, which just tells you brightness of an object, from any given direction.

To get an accurate idea of how a light will illuminate a room, you need to know its output in lumens, and have a diagram showing its brightness in candelas over the range of possible angles it can shine. These two facts allow you to calculate the actual energy falling on an object at any given location in a room, this value is called the lux.