Firstly, it depends on how much light it produces. This is called the Radiant Flux. You will rarely read about this measure because…
It also depends on the colour of the light. Some colours seem brighter to the eye than others, so although exactly the same amount of light is being generated, the apparent brightness is different. This subjective measure is the Luminous Flux, and is the normally-quoted figure, measured in Lumens.
That’s how bright the bulb is: the number of lumens it emits.
But you are as likely to be concerned about how much light it casts on the thing you are lighting. This is called the Illuminance, and is measured in Lux.
One lumen is the light needed to illuminate a square metre at a brightness of one lux.
The brightness of the pool of light from a bulb depends on how big an area the bulb is lighting up. So it depends on how wide or narrow the beam of light is, and also on how far away the bulb is.
A floodlight has to light a much bigger area than a spotlight, so for a given light output, the spotlight can be brighter, simply because its light is more concentrated.
You should normally aim at something between 50 lux for leisure/sitting areas, through 500 to 750 lux for offices up and 1000 to 1500 lux for close, fine work. A bright cloudy day is about 1000 lux. Bright sunshine is about 50,000 lux.