If you plan to typeset beautiful Mathematics textbooks, you will need to know the TEX codes for lots of neat symbols; this is a sort of meta-symbolism, since the codes just tell you how to make a certain symbol in the TEX typesetting language. Learning what the symbols mean is another matter.
The Greek alphabet is easy (for non-Greeks without Classical educations) to recognize as probably denoting special and esoteric entities. This link displays the Greek alphabet and a list of some common symbolic uses for many Greek characters.
The Roman alphabet is harder to recognize as special characters, so in Physics and Mathematics we habitually make use of different fonts for different purposes: we italicize Roman characters that are being used to represent abstract quantities. There are also, however, abbreviations... many of which are single characters, leading to frequent confusion. So we conventionally do not italicize the abbreviations of units. This link attempts to clarify the common uses of italicized and non-italicized Roman characters.