Is it not that terpenes are hydrocarbons while terpenoids can be oxidised (terpenes being the hydrocarbon sub-class of terpenoids)? At least it is the case with carotens and carotenoids
As Catherine has already mentioned, terpenes are hydrocarbons (having just C and H atoms) while, terpenoids are thier oxidized derivatives (bearing others atoms than C and H).
Have a look on the book chapter "terpenoids as main constituents of essential oils" freely available online.