I currently teach in a charter school in florida (
http://www.mckeelacademy.com) ...the way I explain it to people is that it is a public school w/ all of the benefits of a private school. We have to follow state mandated law (standardized testing, etc..), but we don't have to follow the "law" set up by our county school board. We set our own budgets, design our own classes, and ask students to leave if they are not measuring up to our standards. We have higher graduation standards than the other schools in the county.
As a teacher, I love it b/c we are given more freedom with our resources, we are actually encouraged to take our kids on trips and field trips.
A charter school usually is targeting a particular field--for example, our charter focuses on life in the professional world, elementary and ms reinforce that theme and then when the students get to highschool they must choose a career cluster (like a college major) that they will study in for their remaining 4 years. Our school currently offers engineering, journalism, business, culinary arts (only in the middle school), health, and tv/video production. My wife taught at a charter school whose charter was written to help teen mothers. The students would come to school w/ their babies and take family wellness classes along w/ their academic classes.
Each charter school usually sets its own pay scale, teacher requirements, etc. The best thing to do would be use your states doe website.
hope this info helps,
CR