The lux (symbol: lx) is the standard unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry (the science of measuring light), this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface (Wikipedia)
The lumen (symbol: lm) is the standard derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Lumens are related to lux in that one lux is one lumen per square meter. (Wikipedia)
Basically, the higher the number of lumens a light source emits, the brighter it is.