Teaching English as a foreign language (TEFL) involves teaching adults and children whose first or main language is not English. This can be done in the UK or abroad and the students may be learning English for either business or leisure reasons.
Teaching English to speakers of other languages (TESOL) is also a widely used term and often means the same thing as TEFL. It's sometimes specifically used to refer to teaching English to people who are living in the UK but who do not speak English as a first language. These students are most commonly refugees and immigrants and need to learn the language in order to help them settle into UK society. Their courses are often government funded.
Teaching English as a second language (TESL) or teaching English as an additional language (TEAL) may also be terms that are used but they generally all refer to the same thing - teaching English to someone whose native language is not English.
Teachers of English as a foreign language can work in a variety of settings with different age ranges. This can include commercial language schools, schools and institutions of further and higher education throughout the UK and overseas. Some may also teach in industry, while others are self-employed. Classes are usually taught in English, even with beginners.