Latin is quite an interesting language with many words used to represent specific things, for example with "Kill", there are different interpretations...
Here are three different examples of the famous Nietzsche in latin, but be aware there are many versions:
"Quod nos non occidit nos fortiores facit"
"Quod nos non interficit nos confirmat"
"Quod nos non necat nos fortiores facit"
All are technically correct, it just depends on the inflection used in the "Does not kill" section. My latin is not perfect so I am borrowing some information from a thread on wordreference & Allexperts.
Here is the basics of the translation:
That which = QUOD
does not kill = NON OCCIDIT / INTERFICIT / NECAT
us = NOS
makes = FACIT
us = NOS
stronger = FORTIORES.
The whole line does not translate perfectly and is strongly contested online. As it is a popular tattoo idea also I would recommend going to the Latin Discussion Forum which shows possible translations and may be more accurate. Also they discuss the phrase 'What does not destroy me, strengthens me' which may translate better. Find out here:
http://latindiscussion.com/forum/latin/what-does-not-destroy-kill-me-strengthens-me.3280/
http://forum.wordreference.com/showthread.php?t=623371
http://en.allexperts.com/q/Latin-2145/English-latin-confusion.htm
