Hard drugs are drugs that lead to physical addiction, opposed to soft drugs, such as marijuana and hashish, that are only psychologically addictive.
Cocaine is a well-known hard drug, as are the amphetamines, and all the opiates such as heroin and morphine.
According to many, alcohol and nicotine, while freely available for sale, are also hard drugs.
The difference between soft drugs and hard drugs is important in the drug policy of the Netherlands, where soft drugs are dealt with much lighter, up to being for sale as such in clearly recognisable coffeeshops, than hard drugs.
Some consider a few of the hallucinogens to be hard drugs, but this is false by definition, as most hallucinogens are non-addictive. The possible exceptions are PCP, DXM and the amphetamine based empathogens such as MDMA.
This line is further obscured by the drug policy of the Netherlands by classifying synthetic hallucinogens (such as LSD) as hard drugs, even if they have very similar action to naturally occurring drugs such as mescaline, which is considered a soft drug in its natural form of peyote.