Defence potions are made by mixing clean marrentill and then bear fur in a Vial of water, giving 45 Herblore experience. It requires Herblore level 9. They temporarily raise your defence level by 1-8 (1 + 8% of your defence level, rounded down).
Defence potions used to be made with clean ranarr and white berries, requiring a Herblore level of 30 and giving 75 Herblore experience.
Using white berries on a ranarr potion (unf) now yields the message "A defence potion is now made using a Tarromin potion (unf)." As defence potions are now made with a marrentill potion (unf), it appears that this is a textual error. This has been fixed.