HTL or standard Blu-ray discs are the original Blu-ray format. They have an inky brown burning surface and the color lightens after being burned.
LTH or Low to High discs are a golden color and darken with burning.
For both formats the burned information looks the same to the player, so all players can play either format.
Some older burners (older than 2011) will only be able to use the standard (HTL) discs unless they are updated with new firmware.