In ordinary usage, they are synonyms. They both mean beverages in which the alcohol has been concentrated by distillation. Thus, they have a higher percentage of alcohol than you can get by natural fermentation. Brandy is a spirit or a liquor, wine is not. Whiskey is a spirit or a liquor, beer is not. I suspect that "spirits" and "liquor" may both be short names for "spirituous liquor," a term that is never used in everyday life, but which is used in legal documents. "Spirits" always means an alcoholic beverage, while "liquor" has a range of meanings that include non-alcoholic solutions. For example, if kale is cooked by boiling, the water left in the pot, which has been flavored by juices from the kale, is called "pot liquor." However, if someone says they are "going to the liquor store," they are buying something alcoholic! In the United States, the law varies from state to state, and often from county to county or city to city. There may actually be specific legal definitions of terms like "spirits." Driving around the US you may notice that many liquor stores in one locale may have names like "[So-and-so] Wine and Spirits," in another place they may tend to be called "[So-and-so] Liquor Store," and in another "[So-and-so] Package Store." These relate to local laws. For example, where I live there are specific laws for stores that sell alcoholic beverages in sealed containers (rather than serving them in glasses). The law says "package store" so many liquor stores use "Package Store" in their name.

That is an excellent question. Liquor and spirits are generally interchangeable (they mean the same thing). Some people say that a "spirit" is a distilled alcohol, but most native speakers do not recognize a difference. "Liqueur," however, is a sweet tasting spirit, such as an Amaretto.

Note that 'liquor' is less commonly used in British English.

In ordinary use, they are synonymous. Both are very similar and average drinks in which the alcohol has been concentrated by distillation. Therefore, if you have a higher percentage of alcohol than you can get by natural fermentation. However, in Mexico as in other countries, this term is used to make one or more bottles with more or less alcohol. Brandy is a liquor, wine is not. Whiskey is a liquor, wine is not. Whiskey is a liquor or liquor, beer is not. is a term used to differentiate beverages with a greater amount of alcohol than other beverages.