Radicals are a special type component that were originally used to categorize characters in paper (manual) dictionaries.  Every character has a radical (sometimes two radicals) that indicate where in a dictionary a character can be found.  The Chinese word for radical, 部首, literally means section header. It’s the equivalent of an English dictionary that is sectioned off by the 26 letters of the alphabet.

The process of using a character’s radical to search for a character in a paper dictionary is rather cumbersome (involving multiple lookup tables to get to the correct page in the dictionary).

Modern digital dictionaries (either on mobile devices or web-based) obviate the need to use radicals to lookup a character (or word) – most dictionaries will accept drawn characters (best on smart phones), pinyin, characters entered via input method editors (IME’s) and English equivalents).

So, the original use of radicals is obsolete. However, radicals are the most frequently used components – so, understanding radicals as a subset of components is helpful in terms of learning pattern recognition.