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 University). 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

semicolons

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)

commas

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
use commas to set off nonrestrictive elements

"For camp the children needed sturdy shoes, which were expensive."

use a comma to set off transitional and parenthetical expressions,absolute phrases, and contrasted elements
use commas to set off nouns of direct address, the words yes and no, interrogative tags, and mild interjections
use commas with expressions such as he said to set off direct quotations
use commas with dates, addresses, titles, and numbers
use commas to pervent confusion (Hacker 235-245)

Links

OWL Online Writing Lab [(http://owl.english.purdue.edu.)]

Works Cited

Hacker,Diana. A Writer's Reference. 5th Ed. Boston: Bedford/St. Martin's, 2003
"Using Commas." OWL.1994-2004 Purdue University Online Writing Lab. 12July.2006 <htt://owl.english.purdue.edu>