The Basics
Let's start at the beginning. 35 U.S.C. 102(a) is a fairly straightforward statutory bar, although it does have a few subtleties to it. Basically, your application will be rejected under 102(a) if the invention you are trying to patent was already known or used by others in this country. In addition, the invention cannot have been patented or described in a printed publication here in the U.S. or even in a foreign country.

These statements encompass quite a bit of potential prior art. To help clarify, if a patent for the same subject matter as yours (or the applicant) has been granted anywhere in the world, the application will be rejected under 102(a). In addition, even if the invention was just mentioned in a publication anywhere in the world before the application was filed, the invention will be barred from gaining a patent. If the invention was publicly known about or used in the U.S., then it will also be barred from obtaining a patent.

The Details
The terms "known or used" are a little general. To clarify, the PTO interprets these terms very specifically. "Use" refers to any imaginable use of the invention. However, there is one exception, and that is to a secret use or secret knowledge by another person. The "use" or "knowledge" must be publicly available (and not be kept secret) for it to qualify as a bar under 35 U.S.C. 102. Therefore, if Joe secretly uses his new battery recharger in his garage and let's no one else know about it, the invention will not be barred under 102(a).

What may or may not be considered a "printed publication" in regards to prior art? The PTO takes this to mean any item that is provided in an easily reproducible form, even if no one has ever looked at it. This means that a thesis shelved away in a quiet, dusty corridor of a library out in the middle of nowhere will count. If a graduate student thinks up the same subject matter as you years or even months before you apply for a patent, and they describe it in their thesis which is indexed and placed in the library; you will receive a rejection under 102(a).

It gets even worse. An orally presented paper may count as a printed publication too, if it is presented in a public forum and written copies are given out. Leaving nothing for the imagination, the PTO has predetermined that between 50 and 500 people must attend the public forum and at least 6 copies of the paper must be given out for it to qualify as a printed publication.

There is yet another key word to note in 102(a). This section states if the invention was known or used "by others", a patent cannot be granted. To clarify, "by others" refers to any entity different from you. Therefore, if you and your buddies Joe and Frank publish a paper in the U.S. on making porcelain and you later apply for a patent by yourself on the same method for making porcelain, the publication will be considered "by another". The point is, the inventive entity is different even when only one person is missing or added to a reference