The Terra Set consists of all Items that give protection from Earth Damage are have Terra on their name. It can be very useful for lower level mages since wearing this full Set will give you 37% Earth protection (21% without the Terra Amulet), though it also increases Fire Damage by 39%.
Despite the name, there are many other equipment that provide better Earth Damage protection, For example, the Earth Protection Set.
For a list of all Earth Protection equipment, check this page. You can also check the Armor Calculator to compare different set combinations with accuracy.
