Shapeshifting Potions (SSP) change your character's appearance and provide stat bonuses and resistances while active.
Most SSPs are obtained by shapeshifting into one form and then farming monsters in a specific area to obtain the next SSP in the chain.
Some data appears slightly incorrect in OSMW, especially in relation to Ghoul/Wisp. Updates are in progress.
In the very back of the guild HQ you can find an NPC that allows you to upgrade SSPs. To do this, you must provide the same type of ssps for the level you are upgrading.