I agree with Lnafziger's post with the advisory circular definition of holding out versus common carriage.
"AC120-12A - Private Carriage Versus Common Carriage of Persons or
Property contains guidance that can be used to help determine whether
or not an operation falls under common carriage. Take a look at it for
specific examples, but the "plain language" version is:
Common Carriage: A carrier becomes a common carrier when it "holds
itself out" to the public, or to a segment of the public.
Holding Out: A carrier is holding out when they represent themselves
as willing to furnish transportation within the limits of its
facilities to any person who wants it.
There are lots of ways to "hold out", but the basic idea is if someone
in the general public comes to you and asks you to fly them and you
agree, you are probably holding out. Any form of advertising
demonstrates a clear sign that you are holding out. There are also
other activities that qualify, so look over the AC if you have further
questions, and if that doesn't answer it your best bet is to contact
an aviation lawyer. You could also contact your local FSDO with
HOWEVER, and this is the big but that can get you into a world of trouble, just because you have a commercial pilots license and yes you can get legally paid to fly you really cannot carry passengers for compensation. Typically a commercial pilot can have a job like flying skydivers, crop dusting, aerial photography and (with an instructors certificate) provide flight instruction for compensation.
There seems to be a big misconception though that a person can just get a commercial pilots license and take their friends up for compensation. This is false. FAR 1.1 clearly defines commercial operator as:
Commercial operator means a person who, for compensation or hire,
engages in the carriage by aircraft in air commerce of persons or
property, other than as an air carrier or foreign air carrier or under
the authority of Part 375 of this title. Where it is doubtful that an
operation is for "compensation or hire", the test applied is whether
the carriage by air is merely incidental to the person's other
business or is, in itself, a major enterprise for profit.
In fact, inside of AC120-12A itself it even states under Part I:
In summary, persons intending to conduct only private operations in
support of other business should look cautiously at any proposal for
revenue-generating flights which most likely would require
certification as an air carrier.
So while you can fly for hire, you cannot carry passengers or property because then you are acting as a commercial operator. You can work for a commercial operator with that license although for 135 you will need 1200hrs to be PIC. You can do other for hire flying like carrying skydivers, crop dusting, banner towing or aerial photography though as perscribed in the only exceptions stated in FAR 119.1(e).
If you want to fly with your friends you cannot charge them for your time. You could be flying to a business meeting and happen to be on the clock anyway but you cannot specifically charge for the transportation. What you also can do (even as a private pilot) is share costs. FAR 61.113 says:
A private pilot may not pay less than the pro rata share of the
operating expenses of a flight with passengers, provided the expenses
involve only fuel, oil, airport expenditures, or rental fees.
To summarize a commercial pilots license allows you to fly for hire but does not allow you to act as a commercial operator and carry persons or property other than as an air carrier. You can work for someone who has that certificate but you can't just rent or buy a plane and charge a passenger for your time as pilot legally even as a commercial pilot. You can only split costs with them just like you could as a private pilot.
Traps For The Unwary: Business Flying And The "Compensation Or Hire" Rule
Now I have seen some mentions of trying to get away with Part 91 Subpart K as well. § 91.1005 Prohibitions and limitations.
(a) Except as provided in § 91.321 or § 91.501, no owner may carry persons or property for compensation or hire on a program flight. FAR 91.501, however, is going to say this:
(b) Operations that may be conducted under the rules in this subpart
instead of those in parts 121, 129, 135, and 137 of this chapter when
common carriage is not involved, include—
(1) Ferry or training
(2) Aerial work operations such as aerial photography or
survey, or pipeline patrol, but not including fire fighting
(3) Flights for the demonstration of an airplane to
prospective customers when no charge is made except for those
specified in paragraph (d) of this section;
(4) Flights conducted by
the operator of an airplane for his personal transportation, or the
transportation of his guests when no charge, assessment, or fee is
made for the transportation;
(5) Carriage of officials, employees,
guests, and property of a company on an airplane operated by that
company, or the parent or a subsidiary of the company or a subsidiary
of the parent, when the carriage is within the scope of, and
incidental to, the business of the company (other than transportation
by air) and no charge, assessment or fee is made for the carriage in
excess of the cost of owning, operating, and maintaining the airplane,
except that no charge of any kind may be made for the carriage of a
guest of a company, when the carriage is not within the scope of, and
incidental to, the business of that company;
(6) The carriage of
company officials, employees, and guests of the company on an airplane
operated under a time sharing, interchange, or joint ownership
agreement as defined in paragraph (c) of this section;
(7) The carriage of property (other than mail) on an airplane operated by a
person in the furtherance of a business or employment (other than
transportation by air) when the carriage is within the scope of, and
incidental to, that business or employment and no charge, assessment,
or fee is made for the carriage other than those specified in
paragraph (d) of this section;
(8) The carriage on an airplane of an athletic team, sports group, choral
group, or similar group having a common purpose or objective when there is
no charge, assessment, or fee of any kind made by any person for that
and (9) The carriage of persons on an airplane operated by a person in the
furtherance of a business other than transportation by air for the
purpose of selling them land, goods, or property, including franchises
or distributorships, when the carriage is within the scope of, and
incidental to, that business and no charge, assessment, or fee is made
for that carriage. (10) Any operation identified in paragraphs (b)(1)
through (b)(9) of this section when conducted— (i) By a fractional
ownership program manager, or (ii) By a fractional owner in a
fractional ownership program aircraft operated under subpart K of this
part, except that a flight under a joint ownership arrangement under
paragraph (b)(6) of this section may not be conducted. For a flight
under an interchange agreement under paragraph (b)(6) of this section,
the exchange of equal time for the operation must be properly
accounted for as part of the total hours associated with the
fractional owner's share of ownership.
So if you think that you can fly as a "private carrier" under Part 91 and charge for it and not be found as needing a 135 air carrier certificate (or maybe 121, 125, etc..) you are in for a world of hurt. AOPA Legal Counsel Offers a nice write up on some of the differences between having a CPL and being a commercial operator. The exceptions to this rule are the ones in 119.1(e) as mention before (aerial photography, etc..).
Why "private carriage" does NOT relieve you of getting an air carrier certificate. Even if you are found to be private carriage FAR 119.23(b) clearly states:
(b) Each person who conducts noncommon carriage (except as provided in §91.501(b) of this chapter) or private carriage operations for
compensation or hire with airplanes having a passenger-seat
configuration of less than 20 seats, excluding each crewmember seat,
and a payload capacity of less than 6,000 pounds shall—
(1) Comply with the certification and operations specifications
requirements in subpart C of this part;
(2) Conduct those operations in accordance with the requirements of
part 135 of this chapter, except for those requirements applicable
only to commuter operations; and
(3) Be issued operations specifications in accordance with those
Guess what Subpart C of 119 is going to say:
§119.33 General requirements. (a) A person may not operate as a direct air carrier unless that person—
(1) Is a citizen of the United States;
(2) Obtains an Air Carrier Certificate; and etc..
So any notion that you can fly for compensation other than exceptions in 119.1(e) is FALSE unless you also possess the appropriate operating certificate.