In-Game Description Edit
Visitors are people that are not living in the colony. They will typically come to the base, use services and then pay for them with Coins. Once you have good security in place, allowing visitors can be a great source of income.
Although the in-game description specifies that Visitors may only arrive through a Starport, Visitor Ships can still land at Landing Pads.
There is a significant chance that Visitors are actually armed Intruders and will attempt to kill your colonists. Having sufficient Guards carrying guns will mitigate the chance of Intruders doing significant damage to the colony.
Note: Armed intruders will be arriving periodically, as long as you have an active landing pad or starport. Disabling "Visitors" in landing permissions will not prevent them from coming. If you do disable visitors, you can use this to detect attacks early. Visitors, including armed intruders, have purple markings and will show up as "Visitor" when you examine them. Unlike real visitors however, intruders don't have sleep, hunger or bad morale etc.