'
The apostrophe can:

1) substitute for a word/space in a contraction.

2) show possession when placed at the end of a word and followed by 's', save for words that end in 's' or the word 'its'.

3) be used instead of a 'g' in the progressive tense (is doing, was doing, etc.).
1) He's climbing pretty rapidly.

2) Stephanie's shorts were too small, but nobody complained.

3) I'm checkin' e-mail.