Is it illegal to record a conversation without the other person knowing?

All depends on exactly what you intend to use the recording for. If to be used against them in any legal manner, its illegal especially if done without their knowledge and would be tossed out in any court of law. But if you plan to use for personal reasons other than legal manners, this is a sure fire way to lose friends and make enemies

No, at least not for the president who is spying on people.