An inventor can get a patent for a product, such as a machine, which he invented, but also for a specific method of doing something. When a product is patented, no one else may make, use or sell that product. Typically the patent does not describe a product in great detail, but only lists the essential features. This makes it impossible to "work around" the patent by removing some non-essential elements from the invention.
This definition of what exactly constitutes the invention is given in the claims. You can only infringe on a patent when your product matches one or more of the claims. So, if a claim were to say that the invention has four wheels, and it would be possible to build and use the invention with only two wheels, then that two-wheeled variant does not infringe on the patent. For this reason, claims are written in very broad language to cover as many variations as possible.
When the patent describes a method, no one else may use that method. In addition, everything produced directly by that method is also covered by the patent. For example, Unisys holds a patent to the GIF algorithm for making images. In the last few years, they have started to enforce their patent by demanding that people buy licenses or stop creating GIF images. As the patent covers the method of making such images, the images produced by that method are also covered by the patent. So, if you create a GIF image with a program that does not have a license, you may not use that GIF image.
In the USA, everyone, including private persons, can infringe on a patent if they make, use or sell the patented invention. However, in general it is too expensive to track down and prosecure individuals who do this. In most European countries, private persons cannot infringe on a patent, as the patent laws there only reserve the patent rights for "use in commerce".
A person or business can only infringe on a patent if the patent is valid in the country where they make or use the invention. So, a German company is free to produce an invention which is patented in the USA. If the inventor in the USA wanted protection in Germany, he would have had to apply for a patent there as well. He can only do this within a year of his original application in the USA.
The World-Wide Web makes this situation a lot less clear. If the US patent covered a method, and the German company built a Website which executes that method for everyone, then any US citizen using that Website is violating the patent. The German company may then be seen as assisting in this infringement, but it would be difficult to bring them to a US court.
More information on patents, copyright, trademarks and other Internet-related law is available on Iusmentis.com.