Most writers think of semicolons as strong commas, when in reality they are more like weak periods. Semicolons are used when separating two independent clauses. (More on independent clauses)
Since semicolons serve the same role as periods, why use one over the other? Generally, a semicolon should be used anytime a conjunctive adverb is used (however, therefore, nevertheless, moreover, consequently, etc.); the use of these conjunctive adverbs shows an inherent connection between the two independent clauses.
Semicolons should also be used between independent clauses with no linking words,
especially if the first independent clause leads to the second.
While not as frequent, semicolons should also be used to separate lists that contain
additional information. (In other words, if there are nonrestrictive clauses in
a list, use a semicolon.)
Note: Unlike the use of a period, the beginning letter of the word following a semicolon should not be capitalized.