Vegetative growth is the second stage in the life of a plant after it completes germination and begins photosynthesis. During this stage a plant will be photosynthesising as much as possible to grow as large as it can before the onset of the flowering (Generative) phase. In essence it is the period of growth between germination and the beginning of sexual maturity characterised by flowering.
All plants have a vegetative stage where they are growing as fast as possible. It is almost standard practice to grow Cannabis plants with no dark period, and increase the speed at which they grow appreciably. Plants can be grown vegetatively indefinitely (Mother Plants for clones). It is up to the gardener to decide when to force the plant to flower.