The slash (/) is also known as: forward slash, stroke, oblique. You should use the slash with care in formal writing.

1. A slash is often used to indicate "or":
Dear Sir/Madam (Sir or Madam)
Please press your browser's Refresh/Reload button. (Refresh or Reload)
The speech will be given by President/Senator Clinton. (President Clinton or Senator Clinton)
Mary will eat cake and/or fruit. (Mary will eat cake, or Mary will eat fruit, or Mary will eat cake and fruit.)

Do not over-use the slash to indicate "or". It can suggest laziness on the part of the writer. The "and/or" construction is widely considered to be very bad form.

2. Use a slash for fractions:
1/2 (one half)
2/3 (two thirds)
9/10 (nine tenths)

3. Use a slash to indicate "per" in measurements of speed, prices etc:
The speed limit is 100 km/h. (kilometres per hour)
He can type at 75 w/m. (words per minute)
The eggs cost $3/dozen. ($3 per dozen)
They charge £1.50/litre for petrol. (£1.50 per litre)

4. People often use a slash in certain abbreviations:
This is my a/c number. (account)
John Brown, c/o Jane Green (care of)
n/a (not applicable, not available)
w/o (without)

5. A slash is often used in dates to separate day, month and year:
On credit card: Expires end 10/15 (October 2015)
He was born on 30/11/2007. (30th November 2007 - BrE)
It was invented on 11/30/2007. (November 30th, 2007 - AmE)

6. The slash is used to separate parts of a website address (url) on the Internet, and to separate folders on some computer systems:
http://www.englishclub.com/writing/punctuation-slash.htm
file:///Users/mac/tara/photos/image.jpg