Brick

The term "bricked" refers to nonfunctional device states. Though nominally meaning that the device is permanently damaged, in practice it includes conditions which range from trivially recoverable (a failed update) to completely unrecoverable (in certain cases involving damaged baseband memory). A phone may be referred to as "bricked" if it will not boot, will not respond to input, will not make calls, etc. Early unlock solutions frequently resulted in partially-bricked phones after firmware updates were applied. Generally speaking, the iPhone is quite difficult to brick permanently via software methods, and in almost all cases the damage can be reversed. These safety factors do not necessarily apply if you modify your bootloader and/or baseband.

Irreversible Software Bricking

The only way to irreversibly brick your iPhone in software is to flash an invalid bootloader to the baseband. All other bad flash scenarios are recoverable some way or another. It is impossible to brick your phone simply by jailbreaking it, since DFU mode will still be available to recover from a bad flash.