A Rough Guide to Punctuation

Good punctuation is crucial for successful academic writing. Many students use little punctuation in their essays beyond commas and full stops. But to be restricted to just two forms of punctuation mark, when writing your essay, is like building a house using only a hammer and a saw: you can do it; but not very well. By learning to use more, or all, of the available forms of punctuation you will be able to communicate and express your ideas, and arguments, more clearly. 

1. Full Stop ( . )

Full stops have three distinct uses: 

- To mark the end of a sentence

- To indicate abbreviated words
  A full stop indicates an abbreviation, unless first and last letters of the word are shown

- To punctuate numbers and dates 

Examples:

- The cat is completely black.

- The teacher will be Mr John Smith (B. Sci.).

- All assignments should be submitted by 6. 6. 04.

2. Colon ( : )

 A colon can be used:

- to indicate that a list, quotation or summary is about to follow; 

- to separate an initial sentence/clause from a second clause, list, phrase or quotation that supports  the  first in a particular way. 

Examples:

- Buy these things: a packet of peanuts, two loaves of bread and a kilogram of steak.
- Writing the assignment is not easy: to begin with you have to do a lot of research.

3. Semicolon ( ; )

   Separates two complete sentences that are, however, closely linked.

   The semicolon can be replaced by a full stop, but the direct link between the two parts is lost. 
Example :
To err is human; to forgive, divine.
Don't go near the lions; they could bite you.

4. Comma ( , ) 
Commas have a vital role to play in longer sentences, separating information into readable units. A single comma ensures correct reading of a sentence which starts with a longish introductory element. Pairs of commas help in the middle of a sentence to set off any string of words which is either a parenthesis, or in contrast, to whatever went before. 

Example: 
When Australia celebrated its sesquicentenary in 1938, there was a little of the confidence or enthusiasm of the centennial celebrations of 1888. 

5. Question Mark ( ? ) 
A question mark is used at the end of a sentence which is a question. 

Example:
Have the students completed the exam? 

6. Apostrophe ( ' ) 
 There are two uses for the apostrophe:
1. Contractions
A contraction is a shortened version of a word. An apostrophe is used to show that something has been left out, and where it has been left out.
2. Possessives
An apostrophe is used to indicate ownership/possession with nouns. To show ownership by a single individual, insert the apostrophe between the noun and the 's'. To show ownership by more than one individual, use the apostrophe at the end of the word.
Be careful: It's is the contraction of it is. It's is not a possessive (a possessive denotes ownership). 

Examples :
don't (do not)
It'll (It will)
she'll (she will)
the dog's tail (belonging to a single dog)  
the women's magazines
boys' football boots (belonging to more than one boy)
Einstein's theory of relativity
Avagradro's number 

7. Hyphen ( - )
 When used correctly, a hyphen links two or more words, that normally would not be placed together, in order that they work as one idea and these are called compound nouns. 

Example:
Stonier's post-industrial economy is a service economy.
There are four types of information-related machines

8. Parentheses ( )
Parentheses are brackets used to include extra or nonessential material in sentences.
Parentheses should be used sparingly and always appear in pairs. In citation systems like Harvard, parentheses are used to include in-text references.

Example:
It was unusual to see Paul awake so early (as he often studied late into the night) and Jane greeted him with amazement.

9. Exclamation Mark (!) 
An exclamation mark is used at the end of a sentence and indicates surprise, anger, or alarm.
Exclamation marks should be used very sparingly and are not often used in academic writing. 

Examples:
The police stormed in and arrested her!
How disgraceful! 

Sources :http://www.lc.unsw.edu.au/onlib/punc.html