Why does everybody use Yes/No/Cancel buttons?

Typically, "Yes" or "No" might be the simpliest and most straight forward approach. "Yes" being a positive action, "No" being the negative, and "Cancel" allows the user to back out without taking action on the form.

1. Why cancel button is used?
2. What will happen if you choose OK instead of cancel in this dialog box?
3. Should cancel buttons be on the left or right?

Why cancel button is used?

Cancel is mainly useful for multi-step dialogs where the user has progressed past one or more pages with actions. At this time, pressing the Back button will not undo these actions and it would be better if the user would click Cancel.

What will happen if you choose OK instead of cancel in this dialog box?

Listing OK last improves the flow, because the dialog box "ends" with its conclusion. Also, as with Previous/Next, you could argue that OK is the choice that moves the user forward, whereas Cancel moves the user back.

Should cancel buttons be on the left or right?

In most languages, people are accustomed to read from left side to right side, so it accords with our speech habits to put ok-key on the left side of cancel-key. 2. “Ok/cancel” type make it easier to see and select ok-key.