A stalactite is a type of formation that hangs from the ceiling of caves, hot springs, or even structures such as bridges and mines.
A stalagmite is a formation that occurs when stalactite drip to the ground, creating columns, rising from the floor of a cave. They often unite with the stalactite.
Easy to know the difference by remembering a stalactite holds 'tight' to the ceiling and a stalagmite grows 'mighty' from the ground.
Cave formations take millions of years to form.