In SharePoint PreSaveItem function is called when onClick event handler is fired for Save button in List Forms.

PreSaveItem function:

function PreSaveItem()
{   
   if ("function"==typeof(PreSaveAction))
   {
      return PreSaveAction();
   }
   return true;
}

PreSaveAction function allows to override behavior for a Save button. It is commonly used for client side validation, for example:

function PreSaveAction() {   
   return ifValidSSN();
}

In SharePoint PreSaveItem function is called when onClick event handler is fired for Save button in List Forms.

PreSaveItem function:

function PreSaveItem()
{   
   if ("function"==typeof(PreSaveAction))
   {
      return PreSaveAction();
   }
   return true;
}

PreSaveAction function from another hand, allows to override behavior for a Save button. It is commonly used for client side validation, for example:

function PreSaveAction() {   
   return ifValidSSN();
}