Revelle Humanities Grammar Handbook
6 Sentence fragments
7 Run-on sentences
8 Verb agreement
13 Passive voice
15 ESL
17 Misplaced modifiers
18 Pronoun reference
20a Comparisons
21 Parallel construction
23 Commas
23d Sexist language
25/6 Semicolons & colons
29 Ellipsis
31 Brackets
35 Apostrophes
36 Abbreviations
37 Underlining
39 Numbers
41b Slang
41e Jargon
41f Pretentious language
42c Idioms
42e Clichés
43a Fluff

25 & 26 Semicolon and colon
Semicolons are used to separate main clauses; consequently, you can use them instead of a period in order to show a close relation between the two main clauses (as in this sentence).

Colons are used in the following instances: to introduce long quotations; to introduce lists; and to separate two main clauses (except as above when a semicolon is preferable).

Do not use a semicolon when separating a main clause from a dependent clause

Incorrect: Penelope barely recognized Odysseus; because he was away for twenty years.

Revised: Penelope barely recognized Odysseus, because he was away for twenty years.

See your English Handbook for other instances of when and when not to use colons and semicolons.