They are public schools which work outside the regulations of "regular" public schools to try and provide excellent educational opportunities. Read on for more:

"Charter schools are nonsectarian public schools of choice that operate with freedom from many of the regulations that apply to traditional public schools. The "charter" establishing each such school is a performance contract detailing the school's mission, program, goals, students served, methods of assessment, and ways to measure success. The length of time for which charters are granted varies, but most are granted for 3-5 years. At the end of the term, the entity granting the charter may renew the school's contract. Charter schools are accountable to their sponsor-- usually a state or local school board-- to produce positive academic results and adhere to the charter contract. The basic concept of charter schools is that they exercise increased autonomy in return for this accountability. They are accountable for both academic results and fiscal practices to several groups: the sponsor that grants them, the parents who choose them, and the public that funds them.

People establish charter schools for a variety of reasons. The founders generally fall into three groups: grassroots organizations of parents, teachers and community members; entrepreneurs; or existing schools converting to charter status. According to the first-year report of the National Study of Charter Schools, the three reasons most often cited to create a charter school are to:

Realize an educational vision
Gain autonomy
Serve a special population

Parents and teachers choose charter schools primarily for educational reasons--high academic standards, small class size, innovative approaches, or educational philosophies in line with their own. Some also have chosen charter schools for their small size and associated safety (charter schools serve an average of 250 students).

When I used to live in NYC, charter schools were schools that depended more on parents, teachers and private companies for resources. Sometimes a teachers' salary was higher at charter schools or lower than the salary a teacher will get at a regular schools. Charter schools had different rules or additional rules than the regular schools. They will also focus on specific things. For example, Business or Math or Science.