Part 135 is the section of the Federal Aviation Regulations that provides general operating and flight rules for commercial aircraft receiving payment for carriage of passengers or cargo.
The U.S. Federal Aviation Administration (FAA) in the CFR Title 14 part 135 defines a Part 135 Operator. These are the regulations that define the operation of small non-commercial aircraft within the US, however, many other countries defer to these rules. These rules set conditions, such as weather, under which the aircraft may operate.
Part 135 rules are designed to hold pilots, aircraft, operations and even passengers to a higher standard than would pertain to someone providing his own transportation. Part 91 still applies, unless Part 135 overrides with Part 135 regulation.