"veg" is when the plant grows its vegetative mass. you know, the "support structure", this includes fan leaves for energy conversion and stems for flowers to bud on. in this stage lighting needs to be 18 hours or more, as to better reflect the long days of summer.
"flower" stage is when the plant produces is sexual organs (flowers and pollen sacks) mostly all the vegetative growth is stopped during this stage, and the plants primary focus is reproduction via pollination and seed formation. we however intervene (hopefully) and yank the males before they are mature enough to pollinate the females. thus we end up with copious amounts of sensimillia (that means "without seed").
the end.