Savage Orc Tribes refers to a group of minor Greenskins factions who are grouped together for diplomacy.
They all function differently from a typical Greenskin army, as they typically only field Savage Orc units, and without the need of the Savage Orc Resource in order to recruit them. Additionally, they can function as a Horde army, even though they can take settlements. As a result, they will not take attrition if you destroy all their settlements.
They can also be respawned on the map via an Event.
The Bloody Handz Greenskin faction lead by Wurrzag is Savage Orc focused, but is not considered a full Savage Orc tribe. Currently it is not possible to play as a Savage Orc tribe, only Greenskin tribes.
Savage Orc Factions
Introduced in the The Old World campaign:
Skullsmasherz is not to be confused with Skull-takerz, which is a regular Greenskin tribe and not a Savage Orc tribe.
Skullsmasherz also appear in the Season of Revelation campaign, but it is just considered a regular Greenskin faction here, and has a different leader than in the main campaigns.
Introduced in the Eye of the Vortex campaign:
All three of the Savage Orc Tribes appear in the Mortal Empires campaign.