Typically inventions are aesthetic designs, functional items, functional methods, or asexually reproduced plants. Patents provide the right to exclude others from making, using, selling, offering for sale or importing the invention described in the claims. This is perhaps the most powerful monopoly legally obtainable for products. A patent is national in scope. Patents can be obtained in most foreign countries.
Infringement occurs when one practices the invention described in the claims.
Knowledge, or lack thereof, is generally irrelevant to issues of liability. Independent development is not a defense. Knowledge of the patent by the infringer, however, can be relevant to damages. Many companies do new product clearance searches to avoid law suits. There is a requirement that the invention be completely disclosed.