“I think; therefore, I am” – Descartes
Quotation marks – or quotes or inverted commas – are the things put around words to show that other people have said them. They show things that have been (or will be) said in another place or another time, marking them as something which requires special attention.
Quotation marks come in singles (‘___’) or doubles (“___”), and they always come in sets of two. In fiction, quotation marks are quite common as they go around all dialogue; in non-fiction they should be judiciously used around quotes to prove a point or support a thesis.