A flower bud is an immature flower which is usually covered in protective scales which shelter it while it develops. As the flower matures, the bud opens, allowing the flower to emerge. This can take hours or days, depending on the climate and the plant. Once fully matured, a flower may also curl up during various times of the day or night for the purpose of protecting itself from harsh conditions. The flower bud is often used in metaphors which are designed to imply potential, because every flower bud has the potential of developing into something beautiful.
If a flower bud does have scales, these scales are actually modified leaves. Depending on the plant, they may enlarge and open with the bud, or they may shrivel and drop away. Some flower buds are covered in a gummy material which offers additional protection while the flower matures, deterring insects which might attempt to set up camp inside the sheltered area of the bud. This sticky material can also cause flower buds to adhere to passing animals, which is one of the many reasons plants tend to produce multiple flowers, ensuring that at least some of them make it all the way to maturity and fertilization.
Other flower buds may be hairy, sometimes retaining the hairy scales as the flowers mature. Some plants may have naked buds which are fully exposed during development. It is not uncommon for flower, leaf, and stem buds to look remarkably similar until they have developed and matured, because plants often employ the same protective techniques for all new growth which is in the process of budding.
Depending on the plant, a flower bud may develop into a single flower or a group of flowers. As many gardeners have noted, if a branch with buds is severed and is well cared for, the buds may mature and unfurl on their own. This is sometimes used by companies which supply cut flowers, with the company cutting the flowers as buds so that they can be transported without damage, and then promoting maturation once the flowers reach their destinations.
Not all flower buds mature. Sometimes a bud becomes diseased, in which case it may fail to open. Infestation with parasites, insects, worms, and other organisms can also delay maturation. Sometimes, an entire tree or plant becomes diseased and none of the buds open, in which case the plant will not produce flowers or fruit.