From Jonathan Gardner's Korean Notebook In Chinese writing (한자),
characters can be classified by a root or parent character. This
is called the "radical". - Example: The character 三 is the same
as the character 一 with two additional lines added to it.
Therefore, this is rather obviously derived from it. Or, in
other words, the radical for 三 is 一. There are only a few dozen
very common radicals. The rest are relatively uncommon, and
you'll learn them as you learn the characters that use them.
When looking up a character, you can look it up by sound or by
the total number of strokes. Both of these can be unwieldy,
particularly when you can easily identify the radical of the
character. In a Chinese character dictionary, the characters are
usually organized by radical, so you simply need to flip to that
section, count up to the number of strokes, and find your
character from among the two or three others that share the same
radical and stroke count.