Within the world of premium sake, any labels that do not explicitly state "junmai" on them can be assumed "honjozo." Honjozo is sake that has a small amount of brewer's alcohol added to the fermenting sake mash.
What does this mean? Many sake drinkers hear the definition and assume that honjozo is a sweeter fortified product with alcohol added during fermentation, or that it's necessarily higher in alcohol than junmai, or that it's inferior to junmai, because alcohol is added. All three of these theories are false.
Honjozo sake is not inherently sweet—the alcohol is generally added after the yeast has completed fermenting the sugar in the sake. Honjozos are not higher in alcohol than junmais. Both honjozo and junmai, with the exception of genshu (undiluted) sakes, are diluted with water to reach a lower alcohol content. Although makers of bulk sake do often add large quantities of alcohol to increase overall volume, honjozo sake producers don't add much. To qualify as honjozo, the weight of the added alcohol must be no more than 10% of the weight of the sake rice used in brewing. In fact, some of the best honjozo brewers use even less.
Why add alcohol?
Sake brewers discovered that certain aroma and flavor properties in the fermenting mash were much more vibrant when a small amount of added alcohol was added. The added alcohol also increases sturdiness and stability, so honjozos can maintain quality longer than junmais both before and after opening the bottle.
The alcohol added to make honjozo sake is referred to as "brewer's alcohol," or pure distilled alcohol. These days the alcohol is most commonly distilled from table rice, although some brewers use alcohol distilled from sake rice, which is said to produce a higher quality of flavor and aroma.
In a blind tasting, you probably couldn't correctly tell a junmai sake from a honjozo every time. The flavor is more more dependent on what rice and yeast were used than on whether a sake is a junmai or honjozo. But honjozos do have certain qualities that stand out. While junmai sakes have a fuller texture, honjozos are much lighter on the palate, so they're easier to drink throughout an entire meal, or an entire evening. Richer junmai sakes can tire the palate after too many glasses.
Some junmai sakes are so layered that ten minutes into drinking a glass, you'll notice aromas and flavors that were nonexistent when the bottle was initially opened. Swirling and even stirring junmai sakes can speed up this process. Honjozos, on the other hand, are generally less complex and maintain the same aroma/flavor profile as the glass sits. For this reason, honjozos are often good candidates for gentle warming, as they do not have the subtleties of junmai that can get lost with the slight raise in temperature.
Some honjozos to look out for include Nyukon Tokubetsu Honjozo* from Niigata, Kira Honjozo* from Hokkaido, and Ban Ryu Honjozo from Yamagata.
Disclosure: sake breweries marked with asterisks are part of the sake portfolio I manage for Southern Wine & Spirits of New York and Lauber Imports of NY