What happens if I reset my device? Will I lose everything?

If you reset no data is lost

If you restore you will lose all your data

A reset = pressing and holding the power/sleep button and the home button simultaneously until you see the Apple logo re-appear on the screen, then let go and allow it to finish rebooting.

A reset will not delete anything.

Restoring an inphone in iTunes will erase everything and replace it with factory defaults or whatever you have in the form of a backup to restore from.

see http://www.apple.com/support/iphone/assistant/phone/#section_1