The Terrestrial Kingdom is an order of magnitude more glorious than the Telestial realm, and will be the eternal home of Church members who lead honorable lives, but were not valiant enough in their testimony of Jesus to meet all the requirements of Celestial glory. Its inhabitants will also include those who received the gospel while being tortured in Hell.
Jesus Christ will visit this realm, but those in the Terrestrial Kingdom will never return to the presence of Heavenly Father. Those with Terrestrial glory will minister to those in the Telestial Kingdom, but like them, they will forever be unable to bear further children, and the family bonds they had during their mortal lives will not continue through eternity.
Also from the creator of The Brick Book of Mormon: