Argumentum ad verecundiam
An argument from authority is one in which a proposition is claimed to be true because an esteemed person says it is true. It is a fallacy in that it relies on the person's fame or reputation, rather than on logical arguments or empirical evidence.
- "Albert Einstein believed in God. Are you saying that Einstein was wrong?"
- "Robert Gentry, a world-famous astronomer, calculated that the odds of life appearing by chance are astronomically low."
- The noted archaeologist Sir William Ramsey, said “Luke is a historian of the first rank; not merely are his statements of fact trustworthy … this author should be placed along with the very greatest of historians.”
- Would you charge the Declaration of Independence with error in affirming that "all men are endowed by their Creator…"? 
- What do you say about the hundreds of scholarly books that carefully document the veracity and reliability of the Bible? 
Not Always a Fallacy
It is not always a fallacy to say that "So-and-so says that X is true, therefore X is probably true." For this discussion, it is necessary to distinguish between an expert and an authority.
If a famous astronomer says that the universe is expanding, then it is very likely that the universe really is expanding. If a qualified doctor says that a patient is suffering from Parkinson's disease, that's most likely the case. In these examples, the astronomer and the doctor are experts in a field, and are addressing topics within their area of expertise. As experts, they have studied their respective fields, are familiar with the state of the art, have studied how to recognize certain events, features or conditions, know how to recognize many problems that might lead a layman astray and how to work around them, and so forth. When we take an expert's word for something, we are saying, in effect, that if we had the time to learn as much about the field as the expert has, we would be able to examine the evidence and reach the same conclusion.
Naturally, this applies only to experts speaking within their field of expertise: there is no a priori reason to take an astronomer more seriously than anyone else on the subject of foreign policy or theology.
On the other hand, if the Pope says, ex cathedra, that contraception is a sin, then that's true as well. In this case, the Pope is an authority in matters of sin: it is his job to determine what is and isn't a sin in the Catholic church. In a very real sense, contraception is a sin not because it is intrinsically bad, or even because it contradicts the Bible in some way, but merely because the Pope has declared it to be so.
Note that in science, there are experts, but (ideally) no authorities.
On Sir William Ramsey
Ramsey was a Christian apologist who set out to Asia minor in order to prove a specific aspect of the Book of Acts. Naturally he would consider Luke a reliable source, otherwise Ramsey would not have set out on a long and expensive trip to prove Luke's assertions. Further, Ramsey provides no evidence for his claim other than Luke was able to accurately name various cities and countries.
On Albert Einstein
In particular, the example of Albert Einstein can be used to clearly point out why this fallacy is so problematic. If one is in conversation with a person using this example, one can ask:
- Was Einstein ever wrong about anything?
- If so, how then do we tell which things Einstein said were false, and which were true?
The answer to that question, of course, is objective investigation with evidence (Science), and thus, the reason why the statement is true or false is not because Einstein said so, but because of the evidence.