What is the difference between primary, secondary and tertiary alcohols?

The alcohol contains functional group -OH. You have to check for the hydroxy carbon i.e. the carbon to which this -OH is bonded to.

If the hydroxy carbon is bonded to only one carbon, then it is called as primary alcohol. Eg. ##CH_3CH_2OH##

If the hydroxy carbon is bonded to two carbons, then it is called as secondary alcohol. Eg. ##CH_3CH(OH)CH_3##

If the hydroxy carbon is bonded to three carbons, then it is called as tertiary alcohol. Eg. ##(CH_3)_3COH##

Note : you do not look for the OH bonded with number of carbons. you look for the hydroxy carbon bonded to number of carbons.