USPTO uses its own scheme US Patent Classification System (USPCS).
EPO and the USPTO work toward a Joint Classification System Cooperative Patent Classification (CPC) based on ECLA that will incorporate the best classification practices of the two offices.
fiveIPoffices is an intiative of the five largest patent offices (Europe, US, Korea, Japan, China), which account for 90% of all patent applications, to streamline their procedures including a common classification.
Guides and Tips for searching:
Patents are a combination of a technical document and a legal document. While they must describe an invention in sufficient detail for someone else "skilled in the art" to recreate it, the language used may not be the same language that you would expect in a journal article on the topic. So, a simple keyword search is unlikely to be comprehensive. A search by patent classifcation number enhances your search result.