WTUser vs WTPrincipal

Hello, I am currently querying users by email.

I am doing it this way - so target is WTUser

querySpec.appendWhere(new SearchCondition(WTUser.class, WTUser.EMAIL, SearchCondition.EQUAL, email,false), new int[]{0});

But I also found this way:

OrganizationServicesHelper.manager.queryPrincipals(...)

Do both solutions go through the entire user list of Windchill since the second one is for principals and not for users?

And what is the general difference between WTUser and WTPrincipal or are they the same?

BR