All good answers but heres the detailed scoop from flasher-for-hire:
There are two types of flash. A full flash and a partial flash. A full flash involves re-writing the phone's operating system (the OS i.e. how you install windows on a computer to use it) to flashable ROM (Read-only Memory). Flashable means although it is called a ROM it is possible to write to it in special cases such as when installing, changing, or upgrading the OS. A full flash does erase all of your stored data (contacts, pics, txts, etc...) and settings, however a professional technician or at least a knowledgable phone geek will know how to properly backup and restore all your data and sometimes even your settings/preferences. The OS comes as what's called a monster pack, for typical mobile phones, or a ROM image for PDA/Smart phones. The phone technician/geek will have software that allows them to access your phone at the hardware level via a usb cable so that the OS can be written to the phone's ROM.
A partial flash is when you basically just reprogram the phone to connect to a different carrier. This can sometimes be as simple as accessing your phones service menu to enter the correct information, but usually it requires flashing whats called a Preferred Roaming List or prl file. This is a list of cell tower IDs that the carrier your looking to use runs their network on. Partial flashes only reprogram your phones technical settings so it connects to your desired carrier. They do not rewrite any system software to your phone. i.e. you have a Verizon phone and I flash a Cricket PRL to it and point its settings to Cricket's network. Bang, your phone is now on cricket, but it still has Verizon's software on it, and still appears to be a Verizon phone. This is why it's called a partial flash. You usually only get part of the functionality you would have if the phone had Cricket's software on it instead of Verizon's. i.e. VZ navigator isn't going to function on Cricket's network and Cricket Clicks app downloads aren't going to be accessible on a phone without BREW (rare exceptions noted). Technicalities aside, partial flashes at least get you Voice and TXT. Most will give you MMS and Internet access as well. Some will give you app download access. Rarely will you get special features particular to a specific carrier (i.e. VZnavigator). As opposed to a full flash where you're installing your carrier of choice's own OS. If a particular carrier doesn't sell the phone in question you can guarantee they will not have their version of an OS available to perform a full flash. This is where the necessity for partial flashes comes from. I've partial flashed PDA's, Pocket PCs, smartphones to Cricket, for instance, and they don't sell a single phone like that (At least not at that time).
1. ANY SORT OF FLASH WETHER DONE BY A PROFESSIONAL OR YOUR DORMATE MAJORING IN CS ALWAYS CARRIES A RISK OF SUDDEN DEATH. THIS IS REFERRED TO AS "BRICKING" A PHONE AS IT RENDERS IT FUNCTIONALLY USELESS AND THEREFORE ONLY STRUCTURALLY USEFULL AS A BRICK.
2. THE CARRIER YOU ARE FLASHING TO HAS TO USE THE SAME COMMUNICATION TECHNOLOGY AS THE CARRIER THE PHONE IS ORIGINALLY FROM. FOR INSTANCE, A VERIZON PHONE CAN BE FLASHED TO CRICKET, BUT IT CANNOT BE FLASHED TO T-MOBILE OR AT&T. THERE ARE TWO TECHNOLOGIES (to my knowledge), CDMA AND GSIM. ALL CARRIERS IN THE USA WITH THE EXCEPTIONS OF T-MOBILE AND AT&T USE CDMA. T-MOBILE AND AT&T USE GSIM AS DOES MOST OTHER COUNTRIES OUTSIDE USA. IT REALLY DOESN'T MATTER WHICH TECHNOLOGY THE PHONE IS EXCEPT AS TO WHAT CARRIERS ARE AVAILABLE TO FLASH IT TO AND WETHER OR NOT YOU LIVE IN THE US BUT TRAVEL INTERNATIONALLY. HENCE THE "MORE BARS IN MORE PLACES" MOTTO OF AT&T (a GSIM carrier). TECHNICALLY SPEAKING CDMA PHONES HAVE HARD CODED ESNs (Electronic Serial Numbers) WHEREAS GSIM PHONES USE A REMOVABLE SIMCARD TO IDENTIFY THE PHONE.
3. YES YOU CAN JAILBREAK AND FLASH AN iPHONE TO T-MOBILE, BUT WHY?
4. FLASHING OF A PHONE BY ANYONE BUT AN EMPLOYEE OF AN AUTHORIZED DEALER/SERVICE SHOP FOR THAT PARTICULAR PHONE WILL VOID ANY WARRANTY IT MAY HAVE.
5. MAKE SURE THE CARRIER YOU FLASH TO ACCEPTS FOREIGN ESNs ON IT'S NETWORK. MOST DISCOUNT CARRIERS WILL LIKE CRICKET OR BOOST. THE BIG BOYS TYPICALLY DON'T APPROVE.
6. DO NOT ATTEMPT TO FLASH AND ACTIVATE A STOLEN/LOST PHONE. YOU WILL NOT ONLY EPICALLY FAIL, BUT MAY EVEN ATTRACT UNWANTED ATTENTION TO YOURSELF.
Hope you made it through this without nodding off :) Let me know if I can help anymore.