Political Insider has the details.
Charter schools teach the same subjects as all other school systems but each in a different manner. They are experimental. That is what makes them special.
Students in a school district have a choice to go to their ordinary public schools, each which teach in a uniform manner not only through the system but in accordance with state policy (until now, at least, since state establishment of policy may now be question), or attend instead the special and experimental charter school.
Yes, charter schools do compete in a sense with local school systems; charter schools also compete with each other. That is neither bad nor unconstitutional.