Illuminance (lux): Illuminance is a measurement of the light intensity at any point. The light intensity drops exponentially the further away you get from the source (distance).
Luminous Flux (Lm): The luminous flux is a measure of the total light output from a source. For example, a 1 candela light source will product 1 lumen per square meter at the distance of 1 meter.
Beam Angle: The beam angle is the angle of radiation for a light source. For example a beam angle on 30 degrees means the light has a shape of a cone and its borders form a 15 degree angle with the center axis. A change in the beam angle affects the luminous intensity (lux) of a light source but not the luminous flux (Lumens).