Stone guardian is a label applied to two groups, or types, of titanic construct. Both are stone constructs that guard things. Stone guardians come in many forms.
Archaedas, Stone Guardian of the Vault, is one of the ancient servants of the titans, a giant of stone crafted to guard the vault of Uldaman. When threatened, he calls on other stone guardians as often as possible.
The other stone guardians that Archaedas calls on likely include the Animated Statues of Archaedas, Earthen Guardians, and Vault Warders that he brings to the fight. Due to the above wording, the term "stone guardian" may possibly be a broad term referring to many types of titanic creations.