Can someone clearly explain to me what merit and demerit goods are?

[cafff]

I'm studying economics and we briefly mentioned merit and demerit goods but I don't completely understand what they are. I know that cigarettes and alcohol are demerit goods, and that education is a merit good but I don't totally understand the definition?

Is a demerit good just a good which has negative externalities? for example smoking has smoke pollution, alcohol is linked to health problems, anti social behaviour etc ?

and is a merit good something which gives a benefit back to society? like education creates doctors, mechanics etc?

Thanks in advance