Turn resistance is the ability of certain creatures to avoid being turned. This resistance is expressed as a number that is added to the creature's hit dice both when determining if the creature is turned and when determining if the turn attempt has hit its maximum for total hit dice turned. Turn resistance may be negative, making a creature easier to turn. Turn resistance stacks with itself, with an unknown cap. There is a practical cap, though, as no one can turn a creature whose effective hit dice exceeds the turner's by more than 4. Thus, with the level 40 cap, there is no additional benefit to (net) turn resistance above +44.
The item property can provide a bonus of up to +50 per property. Decreasing turn resistance can only be accomplished through an effect.
Outsiders benefit from additional turn resistance derived from their spell resistance. If the turner has the planar turning feat, then half the outsider's spell resistance is added to its turn resistance; otherwise its full spell resistance is added. (The latter case assumes the turner is a cleric of the good or evil domains, as otherwise the outsider would not be affected by turning.)
Turn resistance applies only to non-player characters as neither the item property nor the effect affects player characters. If a player character has taken on an outsider race (e.g. outsider shape), the spell resistance will still emulate turn resistance.