While speaking, we pause, stop or change the tone of our voice to emphasise and clarify what we mean. In writing, we miss this advantage. Hence, to make the written text sensible, we resort to using a variety of punctuation marks. They are like traffic signals alerting readers to identifiable units of the text. Innocuous they may look, but its usage often raises doubts in our minds.

Comma ( , )
Separates the items in a list of words, phrases or clauses:
If you listen to me, concentrate and think, you will understand this lesson.

Separates an adverbial clause or a long phrase from the main clause:
When the sun is shining and the birds are singing, the world seems a happier place.

Use it in a sentence that begins with a non-finite or verb-less clause:
To reach the office on time, she left an hour early.

Separates an introductory / traditional phrase (By the way, Therefore, However etc.):
As it happens, however, the witness turned hostile.

Separates a dependent clause that interrupts the sentence: The fire, burning for the last two days, was still blazing fiercely.

Use it before and after a non-defining relative clause or a phrase giving extra information about the noun it follows: Mount Everest, the world's tallest mountain, was first climbed in 1953.

Use it to join two independent clauses with a coordinating conjunction: He was to go trekking, but could not because of rain.

Separates a question tag or similar phrase from the rest: It's quite expensive, isn't it?

Separates discourse markers from the words spoken, except when using a question or exclamation:
'That's all I know', he said.
'That', he said, is all I know.'
He said, 'That's all I know.'

Speech within speech is introduced by a comma and closed by quotation marks:
'When the judge said, 'Not guilty', I was relieved'.

In a group of items, use the comma to separate the items, but not etc. Table, Chair, Pen, Pencil etc.

Colon ( : )
Used after a term describing a group or a linking phrase:
Do it this way: read, write and memorise.

Semicolon ( ; )
Use it to separate parts of a sentence that already has commas:
She wished to be successful, whatever it might cost; wanted to succeed, whoever might suffer.

Exclamation ( ! )
Use it to end a sentence expressing anger or surprise. (Do not add a period to it.):
What a movie!

Hyphen or Dash ( - )
Use it instead of a colon or semicolon to indicate a conclusion:
You have cheated me - how can I trust you again?

Use it to separate extra information or comment from the rest of the sentence:
Winter here - contrary to what you think - can be very cold.
Hyphens & Compound words
Hyphens are needed to clarify the sense:
The phrase crude oil production statistics needs a hyphen to tell the reader whether 'crude' applies to the oil or to the statistics.

Usually run together prefixes, except where the last letter of the prefix is the same as the first letter of the word to which it attaches:
Pre-empt ; Part-time ; Re-election ; Re-entry
Exception: Where double 'r' appears in the middle:
Override ; Overrule ; Granddaughter

Numbers take hyphens when spelt out; fractions too, but not when used as nouns:
Twenty-eight ; Two-thirds
An increase of two-thirds
Slash or Oblique ( / )
Use it to separate alternative words or terms:
Single/Married/Divorced/Widowed (Delete whichever is not applicable)

Italics
Use it less for emphasis, but more with uncommon, non-anglicized words:
Rang Panchami is a festive occasion in India.

Quotations
Use single quotation marks (') for quotations within a main quotation:
He clarified, 'When I talked to the farmers last week, they said, "We refuse to use pesticides.'".

Apostrophe ( ' )
Use it to indicate the possessive:
Horse's ears ; Princess's ring
Exception: With plural nouns and with names where the final "s" is soft:
Women's (not Womens') ; Children's (not Childrens')
Archimedes' principle ; Achilles' heel

Parentheses or Brackets ( ) & [ ]
Use it to separate extra information or comment from the rest of the sentence:
Mt. Everest (8850 metres) is the tallest peak.

Use square brackets to reveal an error as in the original (which is not yours): 'The minister's letter was an appeal to his 'fiends [sic] and fellow citizens'". - Times

Use it to enclose interpolations in a quotation, or to complete missing detail: 'Post independence, nowhere else has Gandhiji been as sensibly portrayed as in Dutt's movie [Munnabhai]'.

To enclose cross references, numbers or letters Our goal is to (1) increase quality, (2) improve readability and (3) maximize sales.

Capitals
Begin proper names, formal titles, names of reputed institutions etc. in upper case:
The Prime Minister ; Minister ; Lt. General
Member of Parliament ; Human Resource Management

Use lower case for non-specifics:
A management practice

Titles and Headlines
For added emphasis, capitalise all words other than prepositions and conjunctions: Portrait of the Artist as a Young Man

Full stop or Period
Use a Period after an abbreviation. Avoid it, if it comes at the end of a sentence:
etc. Discontinue using a Period between/after initials in a proper name. Well-known Acronyms too do not need Periods in between:
Mr. A S Hornby ; Mrs. Preeti P Patel ; Dr. Mistry
NATO ; AIR ; TISCO

Place full stop outside the inverted commas, irrespective of whether they relate to the words quoted or to the main sentence.
'When the judge said, 'Not guilty', I was relieved."
She is attending classes in "health and beauty".

Do not use periods to end numbered or bulleted points in a list of items: The agenda for next week's meeting shall be as follows:
1. Review of weekly performance
2. Bottlenecks and solutions
3. Planning for the coming week

AD & BC
AD comes before; BC comes after. With century, both are used after:
AD 935 ; 350 BC ; 850 BC ; 1950 AD
Dots (…) /Ellipsis (…)
Use it to indicate a hesitant or interrupted speech, a deliberate omission of a part of the text or an unfinished thought. Always use three dots, leaving a space:
His dying words were, 'I was ...'
Give me a break, I mean …

Dates
Be consistent in your style. Do not mix different styles. Stick to one format:
15 August 1947

Time
Never write 6.00 am in the morning or 6.00 pm last night. Correct version is:
6.00 am or 6.00 pm or 0600 Hrs or 1800 Hrs

Consonant
If a consonant comes after a short vowel on adding '-ing' or '-ed' to the root verb, double the last letter:
Travel - Travelling - Travelled - Traveller
Level - Levelling - Levelled - Leveller
Refer - Referring - Referred

Exceptions : Parallel - Paralleled Ø Develop - Developed
Benefit - Benefiting - Benefited
Focus - Focusing - Focused
'-ize' / '-ise'
Both are correct, but use the more accepted spelling among the two:

Analyse (Not Analize or Analayze) ; Criticise (Not Criticize)
Memorise (not Memorize) ; Paralyse (Not Paralyze)
'-or' / '-our'

As in the above case both are correct, but use the more accepted spelling:
Colour (Not Color) ; Favourite (Not Favorite)
Honour (Not Honor) ; Programme (Not program)
Exception: Technical terms or words used in computer programs/language
Letter-writing
Make use of the 'open punctuation' (no comma or fullstop) in letter formats:
Dear Sir

DearMadam

Yours faithfully

In a proper noun - name of a person - fullstops between the intials can be done away with. Who is talking about periods between generations!

Mr. M R Stephen
Miss. Urvi M Desai