What are oscilators used for?

Can anyone explain me what are those, i heard that they regulate, operate and amplify some sort of signals, it was kinda a complicated answer from my teacher and i didn't understand him, and i didn't want him to repeat because i would get the same answer. What are they used for in for example lets say a PC, card identificator(i've seen it at my doctor's ordination the other day, he uses it to log in in our ministry of health database probably. Etc...what are their task, and what are their purpose in a radio.