The Department for Education talk of Qualified teacher status (QTS) as the following:
Qualified teacher status (QTS) is the accreditation that enables you to teach in state-maintained and special schools in England and Wales.
Who needs QTS?
Anyone who wants to teach in a state-maintained school in England or Wales needs to gain qualified teacher status (QTS). To achieve this award, you need to complete a period of initial teacher training (ITT), which will enable you to meet the professional standards for QTS; a formal set of skills and qualities required to be an effective teacher.
How do you get QTS?
There are a number of different forms of ITT, and many more individual courses for you to choose from. You can complete your training at undergraduate or postgraduate level, and even take an employment-based route.
What if you are already an experienced teacher?
You may already have been teaching for some time; in the independent sector, or as an overseas teacher. Regardless of experience, you will still need QTS before you can teach as a qualified teacher in a state-maintained school. If you have the right experience you won't necessarily need to complete a full programme of ITT. There are options including a programme of QTS assessment only and tailored training for overseas trained teachers.