My bad, I didn't notice this. I'll shoot erzo a link as well.
Apparition is a popular method of transportation among skilled wizards, allowing the user to disappear and reappear in another place, sometimes very far away, with an almost instantaneous speed. While the magic is very convenient, it is also considered to be very dangerous and difficult magic and for that reason many wizards prefer not to use it, opting instead for brooms or Thestrals for travelling.
As Shady explained above, the process of apparition is broken into three steps known as the "Three 'D's." The first D, Destination, requires the user to clearly picture the spot they will reappear in their mind. For Kisei, this is when the spot will likely be marked via magic for the jutsu's summoning aspect. The second D, Determination, relates to how the user must move in order for this to work. If the user is indecisive in regards to any single aspect of this jutsu, they become prone to splynching, an occurrence in which body parts are left behind in the starting position. The final D, Deliberation, instructs not to try to disapparate too quickly; it needs to be a measured, patient process.
If the user is able to properly use the Three 'D's, they will be able to successfully perform the jutsu and disapparate into magic, at which point in time they will effectively be summoned to their target location, with some amount of lag time depending on the distance from the destination.
Dark Wizards can use a simplified version of this in order to achieve flight. By partially converting themselves into magic, they are able to passively float and fly around at extremely high speeds, though this will temporarily limit their ability to use proper Apparition.