The short answer
Beer is made from malt, hops, yeast, and water. Each ingredient has its own role. Malt shapes color and body, hops can contribute bitterness and aroma, and yeast drives fermentation while bringing its own flavors. What you taste in the glass is always the interplay of these four.
Malt Is More Than Just Color
Malt is grain that's been allowed to germinate and then dried. How long and how intensely it's heated during this process determines its character. This is how you get bright bread notes or bold roasted notes.
That's why a dark beer often smells completely different from a light one. Color, on the other hand, says nothing about alcohol content. Dark doesn't mean strong.
There are more types of malt than most people realize. The range runs from pale Pilsner malt through caramel malt to nearly black roasted malt. A brewery blends them the way a chef combines ingredients.
That explains a common source of confusion. Two beers from the same category sometimes taste distinctly different, even though nothing about the hops has changed. The difference lies in the malt bill.
Through all of this, the yeast usually remains invisible. It ferments the sugar from the malt into alcohol, contributing its own aromas along the way. This is especially noticeable in wheat beer.
Hop aroma and bitterness are two different things
Hops can be added at very different points in the brewing process. That timing determines what they bring to the beer.
Hops added early boil for a long time and release bitter compounds. Hops added late, or after fermentation, retain their aromatic compounds. During a long boil, these would evaporate.
So the same plant material has two completely different effects. It all comes down to when it enters the process.
With dry hopping, aroma takes center stage. A strongly fruity smell doesn't mean fruit was added, nor that the beer is especially bitter. It can smell intense yet taste mild.
Remember this distinction: aroma comes from the nose, bitterness from the mouth. Both can be adjusted independently of each other. That's exactly what breweries take advantage of.
Your First Beer Tasting
Don't rush out and buy a whole case of different styles. Instead, describe what you like: low bitterness, distinctly hoppy, bready, or fruity. Those few words will get you further in a shop than any long shopping list.
A Helles and a Pils side by side make a great first comparison. Both are pale, yet you'll notice the difference right away. After that, you can compare a wheat beer against them.
Take small samples and use a clean glass. Note the aroma and flavor separately. This is easier than forming one overall judgment, and it'll tell you more afterward.
If you don't like a beer, look for the specific reason. "Too bitter" leads you toward a very different next bottle than "too sweet." And by the way, an unusual style isn't a test you have to pass.
Temperature matters more than many people think. Served ice-cold, almost any beer tastes muted. A few degrees warmer, and the malt and hops come forward. So try a beer slightly less cold before writing it off.
Take your time. Really getting to know two beers will teach you more than eight you won't remember the next day.