Answer: Probably you're speaking of trills? I like questions like this: the kind thing one might be curious about, but is not going to appear on your midterm.

We don't normally write out trills because it's too much work, and it's too hard to read, and finally it's because trills are ornaments. There was time when formal music was much more improvisatory than today: composers just wrote the "main" notes of a piece, and performers added ornamentation according to their own taste and the style they were used to. Being able to ornament gracefully was part of what made a fine musician.

But eventually composers began to indicate where an ornament would be expected, putting signs of one sort or another down as hints to the performer. A trill here, a mordent or turn there. Performers would make of these what they could. But to actually write these out would be tedious. No one wants that. And writing them out would imply that they could be played just one way.

"Tremolo, or "bowed tremolo," usually refers to a quickly repeated tone made by a sort of nervous vibration of the bow, or a similar effect on a wind instrument, or on a piano when it may involve several notes. The frequency is uncertain, just very fast, so it would be hard to write out even if one had the patience to place all those notes. Trills are an alternation between a main note and the note above, and their performance varies. A trill might start slow and speed up, might end with a turn, or not, etc. So it makes sense not to attempt to notate it literally.