The FARs (or Federal Aviation Regulations) are the regulations imposed by the FAA on aviation within the United States.
Many questions make reference to FAR 23, or FAR 91 etc. What are each of these? Where can one lookup the text?
There are several sites on the web, but my favorite is the FAA's own Regulatory and Guidance Library at http://rgl.faa.gov/. If you click the Code of Federal Regulations link on their main page, you can look at current and historical regulations as well as preamble information, links to the dockets, NPRM's, and more. Just click on "By Part" under the "Current CFR" section, and you can browse through each section. If you then scroll down, you will find the two that you asked about:
Part 23 - AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES
Part 91 - GENERAL OPERATING AND FLIGHT RULES
Click the arrow next to each part and it will show links to the full text of each regulation.
The site also includes the following databases: