All of us have been there at one point in time, many of us might not have noticed it though. Have you ever found yourself in the situation in which you had to express an idea about someone doing something, referring to them as 'he' or 'she'? Some write 'he/she', others write 'he or she', others pick only 'he' or 'she'.

This can be annoying sometimes, especially when you're debating a problem with a group (with >=1 ladies in it). I've noticed that they prefer to use 'he', which is pretty uncool if the problem being debated is of a "men vs women" flavour... men are displayed in a bad light by default :-) Here's an example:

"Imagine that the user has accidentally deleted a file, he will not notice the disappearance immediately, and when he does, it can be too late."

Why he? Don't women delete files by accident?

Some try to remedy the problem by writing 'he/she' or 'he or she' - which bloats the text, making it longer and uglier. This is where they come into action.

Follow up:

In English grammar, singular they is the use of the pronoun they and its inflected forms (them, their, etc.) with grammatically singular antecedents, often of indeterminate sex and/or number:

If anyone claims this seat is theirs, tell them they'll have to show their reservation.

Many don't think that 'they' is the right way to go, because 'they' is the plural form, hence it's not appropriate when a specific person is being discussed. But as you can see, it works with males and females, in groups if >=1 people :-)

I hope the Wikipedia entry was helpful, you can use it as a weapon against your colleagues, teachers, siblings and friends. Good luck :-)