In spite of the comma's reputation as the second most feared item of punctuation (after the dreaded semi-colon), the main purpose of these friendly little critters is to make sentences easier to read and understand. Commas are most commonly used:
To enclose parenthetical expressions
"The Erie Canal, along with many other internal improvements, was built during the Jacksonian Era."
To enclose a name or title in direct address
"Excuse me, Professor, but I missed your point about the importance of the Erie Canal."
farm implements, cash crops, etc., were all carried on canal boats."
"Ronald Shaw, Ph.D., wrote Erie Water West: A History of the Erie Canal, 1792-1854."
To set off non-restrictive elements (i.e. words, phrases, or clauses that modify a word whose meaning is already clear)
"A celebrated politician, DeWitt Clinton was a canal supporter."
"The canal commissioners, all sporting their best attire, looked ready to do business."
"This book describes the life of Peter Ploughshare, an opponent of the Grand Canal."
**Note: A restrictive element is a modifier that is necessary to preserve the intended meaning of the sentence. For example, "The commissioners who wore their best suits looked businesslike" requires no commas because it is necessary to know which commissioners looked businesslike.
In a list
"The canal boat was full of corn, cows, chickens, and cotton."
Before a conjunction (and, but, nor, or, for, so, yet) joining two independent clauses
"The Hudson River was always an important waterway, but its prestige grew after the opening of the Great Western Canal."
After an introductory phrase or clause that precedes an independent clause
"Before you conclude your essay, I think you should take into consideration the impact the Erie Canal had on the Civil War."
To indicate tags and interjection
"Yes, I came here via the artificial river."
"Well, actually, I left the supplies in Lockport."
"You know that the Erie Canal was responsible for the United States becoming a superpower, don't you?"