This is the first stage once the plant becomes a seedling. a plant can stay in this stage as long as the grower wishes (indoor) before he decides to flower (unless it is an autoflower strain) . Plants spend a majority of the time focused on root development during this phase. relatively humid conditions during this stage of growth is acceptable. depending on how much space your grow room contains will determine how long you will keep it in this stage. plants can double or triple once put in to flower.so timing is often critical when space is limited. be sure to choose a strain suitable for your grow space or environment.
During this stage plants like mainly blue spectrum lighting. Metal halide lighting is also a very good choice for use in vegetative growth stage however the use of HPS throughout entire grow is good as well and commonly done. You can view some light options here.
During this phase plants will predominately need a good source of nitrogen in your NPK watering/feeding ratio.
once you are ready to switch to next stage. it is called the flowering stage.