What's the difference between a semicolon and a comma?

The semicolon is used to seperate major sentence elements of equal grammatical rank (Hacker250). The word semi means half.

The comma is a valuable, useful punctuation device because it separates the structural elements of sentences into manageable segments (OWL at Purdue Universsity). The comma was invented to help readers. Without it, sentence parts can collide into one another unexpectedly, causing misreadings.(Hacker235)

The Rules

To avoid common misuses of the semicolon. Do not use a semicolon in the following situtations.

between a subordinate clause and the rest of the sentence
between an apposite and the word it refers to
to introduce a list
between independent clauses joined by and, but, or, nor, for, so, or yet (Hacker253)

To avoid common misuses of the comma. Do not use a comma in the following situations.

after a coodinating conjunction
after such as or like
before than
after although
before a parenthesis
to set off an indirect (reported) quotation
with a question maek or an exclamation point (Hacker249)

To aviod unnecessary commas

do not use a comma between compound elements that are not independent clauses
do not use a comma to seperate a verb from its subject or object
do not use a comma before the first or after the last item in a series
do not use a comma between cumulative adjectives, between an adjective and a noun, or between an adverb and an adjective
do not use a comma to set off a concluding adverb clause that is essential to the meaning of the sentence (Hacker246-248)

The Usage

Use a semicolon between items in a series containing internal

punctuation.

between closely related independent clauses not joined with a

coordinating conjunction

between independent clauses liked with a transitional expression (Hacker251,252)

Use a comma between coordinate adjectives not joined by and

before a coordinating conjunction joining independent clauses
after an introductory word group
between all items in a series