Legalities

United States

AM 4w max, Single Side Band (SSB) 12w max Peak Envelope Power (PEP)
Communications (or attempts to communicate) with stations over 250 km (155.3 miles) away are prohibited.
You must use an FCC-certified CB unit in the United States. FCC-certified CB units have an FCC-certified label placed by the manufacturer.
"take-turn" basis, no CB channel is assigned to any specific individual or organization.
You must never talk for more than 5 minutes straight, and you must wait at least 1 minute before starting another communication.
You may not raise the power output of your CB unit, attach any type of power amplifier, or modify the unit internally
You may attach an antenna that increases range

If you have a dual-use, FRS-General Mobile Radio Service (GMRS) unit, you may legally use only the FRS channels unless you have a GMRS license.
The label on the unit or the operations manual should indicate the service the unit is certified for.
You may not replace the antenna
You may operate anywhere in the U.S. and its possessions, it may not be legal to operate outside the U.S.

Must cooperate with other users in the 5 available channels to reduce interference to other users
No user has priority over other users, except if there is an emergency, then you must yield to the person making the distress call
May not operate as a repeater station
May not do store-and-forward packet radio
May not operate as a signal booster
2w maximum

You may not operate an unlicensed radio outside of its designated service
You may not operate a radio that requires a license unless you have a license for the frequency and power you are operating on
You may be fined, imprisoned, and/or the equipment may be confiscated
The FCC makes the rules (FCC part numbers), they enforce the rules (including coordinating searches, arrests, and seizures of equipment) and they operate their own court which is superior to all other courts except the supreme court. Do the math, the cards are stacked against you so dont break the rules.