Teleportation circles were permanent Portals created by conjuration Spells. The teleportation circle appeared on the floor around the caster, or on whatever horizontal surface they stood on, with a diameter of 10 feet. Any creature who stood within the circle was teleported to a designated location that could only be set once. If the caster tried to designate a location that was a solid object, an area they were unfamiliar with and lacked a clear description, or was in another plane, the spell failed; failed teleportation circles were incredibly dangerous, as they could tear apart any creature who walked inside.
The circle was discreet and very difficult to spot. The caster needed to mark the circle in some way to keep creatures from activating it accidentally.