The above answers are correct in general. However, there's a bit more to it than that, especially if you shoot digital or a film format that has very limited dynamic range like slide film. This is because w/ limited dynamic range, you can easily overexpose the bright portions of your image while properly exposing the rest of it. If you've shot digital outdoors on a bright day, you'll run into this between the sky and whatever foreground. If you properly expose for the foreground, you'll very likely overexpose the sky and "blow out" all the highlights there, eg. detail-less clouds, sky turns white instead of blue, bright small/thin objects in the distance, especially w/ sky background might blend into the sky, etc. This can happen while your blacks are still black. This is because the dynamic range of a real life, outdoor scene in bright daylight is much larger than the dynamic range of your digital camera or slide film. Even today's regular color film cannot truly contain all that dynamic range. Good B&W film comes close though. And yes, I'm not including cases where you shoot towards the sun.
Also, even if your film or digital camera can contain all the dynamic range (and there are tricks for this), the output medium you use probably cannot represent it all w/out "cheating" in one way or another. Typically, you would try to compress that dynamic range to fit on paper, on screen, etc. And usually, such dynamic range is no better than your digital camera or slide film.
Considering this and other factors, you may start to see that photography is not about capturing reality in a realistic way, but rather about realizing and presenting a vision of what the photographer sees w/in his reality. IOW, it's all just an illusion, a slight of hand, in one way or another. :wink: Of course, trying to make photos that are commonly accepted fascimiles of reality is a common instance of all that.
Anyway, for more details and a good beginner's lesson on this stuff, see here: