The semicolon is often misused. Ninety-nine percent of the time, the semicolon should be thought of as close in function to the period--most definitely not the comma or the colon. Use a semicolon where you might well use a period. Instead of a period, you can use a semicolon to show an intimate relationship between two sentences, two ideas, e.g., compare/contrast.
The most common application of the semicolon is to use it to link two independent clauses joined by a conjunctive adverb (therefore, however, consequently, furthermore, thus, moreover) or joined by a transitional expression (as a result, even so, for example, in fact, of course).
Here's an example:
"The castle turret was constructed with remarkable speed and quality of workmanship; however, the stone masons were anything but happy with their wages and working conditions."
Note that a semicolon preceeds the conjunctive adverb or transitional expression and a comma follows.
The semicolon can link two independent clauses without a conjunctive adverb or transitional expression, too, so long as the relationship between the two clause is clear, e.g., "The Duke played cards and billiards all day; he drank aperitifs and champaign all night long as he wooed the ladies."
The modern practice seems to be to avoid overusing the semicolon. How much is overuse? That's partly a matter of style. Frequent use of the semicolon, by most readers' accounts, makes sentences thicker, longer, and perhaps more dense.
Addendum: Yes, there is one instance where the semicolon approximates the function of a comma, and that is in an instance where a list offered. Each item of the list has its own internal comma punctuation, so the semicolon is used to in effect be a "hard comma," separating the items of the list, e.g., "He bragged that he was the only person to have visited each and every Paris in the world: Paris, Texas; Paris, Kentucky; Paris, Brazil; and Paris, France." This would be a confusing list, indeed, if only commas were used. This is the ONLY exception to the "SEMICOLON functions most like a PERIOD" Rule.
The most common application of the semicolon is to use it to link two independent clauses joined by a conjunctive adverb (therefore, however, consequently, furthermore, thus, moreover) or joined by a transitional expression (as a result, even so, for example, in fact, of course).
Here's an example:
"The castle turret was constructed with remarkable speed and quality of workmanship; however, the stone masons were anything but happy with their wages and working conditions."
Note that a semicolon preceeds the conjunctive adverb or transitional expression and a comma follows.
The semicolon can link two independent clauses without a conjunctive adverb or transitional expression, too, so long as the relationship between the two clause is clear, e.g., "The Duke played cards and billiards all day; he drank aperitifs and champaign all night long as he wooed the ladies."
The modern practice seems to be to avoid overusing the semicolon. How much is overuse? That's partly a matter of style. Frequent use of the semicolon, by most readers' accounts, makes sentences thicker, longer, and perhaps more dense.
Addendum: Yes, there is one instance where the semicolon approximates the function of a comma, and that is in an instance where a list offered. Each item of the list has its own internal comma punctuation, so the semicolon is used to in effect be a "hard comma," separating the items of the list, e.g., "He bragged that he was the only person to have visited each and every Paris in the world: Paris, Texas; Paris, Kentucky; Paris, Brazil; and Paris, France." This would be a confusing list, indeed, if only commas were used. This is the ONLY exception to the "SEMICOLON functions most like a PERIOD" Rule.