Dream Leaping is the ability to enter and manipulate the dreams of other beings. It allows users to communicate with others inside the dream and to take control and reshape the dream as they desire.
A mortal named Whitaker Berman once designed a machine that allowed him to enter the dreams of others. He used it to enter the dreams of several women and killed after they rejected him, as being killed in dream would kill them in reality. He was killed inside a dream by Prue Halliwell.
When he was a Whitelighter, Leo Wyatt once used his charge connection to Piper Halliwell to enter her dream and say goodbye after the Elders had taken him away. This was the only time this connection was used, which was only possible due to their love.
When her cousins were under attack by Barbas, Penny Halliwell used her telepathy to enter the dream world they were trapped in and was able to manipulate this world to a certain degree, allowing her to fight off Barbas.
When Barbas worked with the Source of All Evil to defeat the Halliwell cousins, the Source greanted Barbas enough power t restore his corporeal form. Barbas became able to put others to sleep and trap them in a dream world to confront them with their fears. Barbas was in full control of what happened in the dream world, allowing him to shape the word as he desired.