Lumen (lm) is the quantity measure or amount of visible light (light output) emitted by a lighting source. Lumens can tell you how bright the lamp is. The higher the number of lumens, the brighter the lamp will be.
Nowadays with LED, you can get more light with much less power consumption. Years ago we used to buy 50W-60W halogen or incandescent light bulbs, now we get the same amount of light or lumens by only consuming 6W-8W. Today we are able to save more than 80% of energy. In the past we also used to buy watts, now it´s best to buy lumens. Here is a an approximate wattage equivalence comparison. Note values might differentiate according to the type of light bulb.
100W - 1500-1600 lm
Now with LEDs 13W-15W
75W - 1000-1100lm
Now with LEDs 9W-11W
60W - 800-850lm
Now with LEDs 7W-8W
50W - 400-450lm
Now with LEDs 4W-6W
As technology improves, more lumens will be achieved using less watts.
Efficacy is the number of lumens per watt. The higher the number, the more efficient the lamp is.
Candela (cd) is the unit of luminous intensity per angle in a particular direction.
Lux (lx) is the amount of light on a surface per unit area. By measuring lux we need to consider the distance between the light source and surface.
How much light is needed depends on the room size, ceiling height, fixture or luminaire, type of lighting, task area or room purpose.