What is the difference between a 401 and a 403 error in the context of Google's HTTP status codes?

A 401 Unauthorized error means authentication is required. You need to log in or provide valid credentials. A 403 Forbidden error means authentication won't help; you don't have permission to access the resource, even if you're logged in. Think of it this way: 401 is 'who are you?' and 403 is 'you are who you say you are, but you still can't access this'.