Whiskey is a name for a particular kind of distilled spirit.
But no two brands of whiskey are the same. It is made from fermented malt, or grain. There are differences in the malt/grain, the preparation of the malt/grain, the precise method of distillation, the casks it is aged in, and how long it is aged. All of these factors affect the flavor, alcohol content, and presence of other compounds which may affect your brain in subtle ways. Whiskeys may be blends of malt and grain distillations. Some whiskeys are distilled multiple times. Many Scotch whiskeys are made with malt flavored by peat smoke.
'Moonshine' whiskey ( made illicitly ) may also contain methanol as well as ethanol. Methanol is very toxic - a small amount can kill you. A smaller amount may cause blindness, or a shocking headache. A very small amount may enhance the inebriating effect of the liquor. For this reason, some makers deliberately include very small amounts.Since the boiling point of methanol is lower than that of ethanol, it can easily be discarded as the first product of the still.
Generally, the term 'hard' is synonymous with 'distilled' - so all whiskey is hard liquor. Perhaps someone you know has a different usage of the word.
By 'harder variety' I'm guessing they meant higher alcohol content. That may taste less palatable to you.
