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. It is impossible to brick your phone by jailbreaking it, since the jailbreak does not affect the low-level recovery modes of the firmware, and hence it is always possible to un-jailbreak via iTunes. These safety factors do not necessarily apply if you modify your bootloader and/or baseband.

Software Bricking

The only way to irreveribly 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.