Sealing wax is used to seal envelopes. It originated in the days when envelopes did not come with glue on the flap, leaving no way to seal them. The answer was to tip a lit candle over the envelope, allowing a few drops of wax too pool at the flap's edge. This was stamped with an inscribed stem, called a seal. Sealing wax contains resin or other components to make it hard and glossy, to better hold the shape of the seal that is pressed into the wax when it is still soft.
In days of old, sealing wax was both a method to ensure that your communication was secure--a broken seal would reveal spies or treachery in your messenger service--and to authenticate the sender. You would know the proclamation came from the king if the king's seal was affixed to it, since only the king or his trusted scribe would have access to the seal.
The advent of gummed envelopes led to a decline in the use of sealing wax, although it was still made and used to add a touch of glamor, particularly on wedding invitations. More recently, automated mail sorting equipment in the postal system is likely to break or strip a wax seal off an envelop, so sealing wax is more often used on the inner envelope of invitations, if it is used at all.
A more flexible form of sealing wax, known as faux sealing wax, purports to be able to survive being handled by sorting machinery. It is slightly rubbery and can bend without breaking, unlike the more brittle resinous wax. Also available are round sealing wax sticks that fit into standard size glue guns.
You can obtain seals for sealing wax in your initials, a single initial, or other designs. Roses, hearts, cats, religious symbols, or hobby-related designs, plus literally hundreds of other images are available. People choose seals that are meaningful to them or representative of them in some way.
Today, seals and sealing wax are mainly used in scrapbooking, to decorate a certificate to be framed, or to emphasize your intent when penning affirmations or prayers.