I think there are two commonly used barrels.The stuff you learn on TERB
Scotch is whisky aged in an oak barrel.
This is from memory from years ago when I cared more about scotch, but as far as I remember scotch is made in recycled barrels (insert joke about cheap scotts here).
Oak barrels come from bourbon, generally and then they also use wine barrels (forget what they are made from).
Oak gives it a vanilla like flavour and the wine barrels a bit more fruitiness.