Don't be sorry for asking questions! That's what we're here for.

A charter school is a sort of experimental school set up within the public school system to try to provide a special environment for kids who may need a different approach to teaching, or those deemed to be "at risk" (say, in a big city where there are gangs or other bad influences). It's not "special education" (as for handicapped kids), but more a controlled experiment for some particular groups of students. Charter schools have varying degrees of success, depending on the goals they were founded to meet.

K usually means Kindergarten (i.e. the year before Grade 1). K-5 would mean a school that has Kindergarten through grade 5.

Public school in the US consists of 12 or 13 years, grades 1-12 or K-12. The lower grades, K-5 or 6, are usually referred to as elementary school. Grades 6 or 7 through 8 or 9 are middle school or junior high, and grades 8 or 9 through 12 are high school. I hear some districts are going back to the old model, where K-8 are "elementary school", and some districts break out middle school a bit differently or combine junior and senior high schools. You may also see reference to "nursery school" which is anything pre-kindergarten, other than mere babysitting.
Cheers,
Bev