Private schools (also known as 'independent schools') existed long before state schools came along, in some cases for hundreds of years. They are not funded by the government. They therefore charge school fees, though many also offer bursaries (fee reduction). They are ‘independent‘ because of their freedom to operate outside government and local government control. Being independent means that private schools are free from government interference in areas such as admissions, curriculum and assessment, although safeguarding and child protection requirements still apply. All independent (private schools) must be registered with the Government and are inspected regularly. Information on the benefit of their independence is available here.
‘Senior schools’ are the secondary part of an independent (private school.) The primary part of the school is generally known as the junior school or prep school or the preparatory school. Prep schools usually go up to the age of 13 in independent (private schools) but in some schools, as in the state sector, go up to the age of 11. Pupils then enter senior school and at the age of 16-18 are generally in sixth form. Many independent (private schools) are all-through schools though are still generally split up into nursery, pre-prep, prep, senior and sixth form.
Sometimes the term ‘public school’ is also used to refer to a private school. However, this term can be confusing as in other parts of the world, such as in the USA, public schools are free to attend and supported by public funds.