chips, as mentioned, are plugged into the PCM tuners plug into the OBD port and upload their programming. tuner generally can read codes, chips can not. tuners are a one size fits all, chips can be custom made
there are three kinds of chips there is the DP style which is proprietary to DP tuner, and then there is the TS style chip which the rest of the "tuners" use. There is also something called the hydra chip (17 positions that doesn't need to be removed to add tunes, i don't know much about this). there are canned tunes, one size fits all, from TS performance and there are custom tunes from DP, Gearhead, Beans, No limit, Swamps, Tony Wildman, PowerHungryPerformace (also does the hydra) etc. The custom tunes are built for YOUR truck your injectors, HPOP, turbo, driving style
I am running TW tunes it cost 450 or 475 several years ago for them. I went with Tony's tunes because reburns were free, so far i have sent it back 2 or 3 times. someplaces charge $150 for a reburn or $25 per tune. there are many different tunes econ (waste IMO), whisper, race, street, extreme, tow, heavy tow, etc. some tuners have HP levels some don't. trans can also be tuned, varying shift points firmness, etc. I have a 6 position chip, i really only use 2. i have gained 2ish MPGs and have gotten better MPGs on the higher HP tunes, your results may vary. I can't stand driving without a chip. PHP offers a 6position chip for $300
you can not stack chips/tuners on a 7.3
on a super duty it is easy to get to the PCM just inside the driver's kick panel
here's a link -it's the first one that came up on youtube.
7.3 PSD - How To: Install a Tuner - YouTube