he/she

This is used when both "he" and "she" can be written in a sentence.
Use "he/she" to avoid excluding one of the sexes (i.e. when you want be politically correct).
Some people thinks one should use "she/he" instead.

However, he/she could also refer to a transexual.

Personally, for in-text references to entities of unknown sex, I prefer the brevity of s/he.