RGH is an acronym for the words, Reset Glitch Hack, repetitive yes, but that is not important. What RGH allows you to do is, basically, make a Jtag out of a non-jtagable, but what does this mean? Basically, you could only jtag consoles that had a specific dash version, 7379, or something like that, which basically meant if you connected to XBL and upgraded the old Blade dash, you lost the ability to jtag your console, now some consoles did connect and got the new dash and were still Jtagable, that is because the patch came out later, I am just trying to give you an approximate time. So if you downloaded the patch, you lost Jtagability, however new experimentation has allowed a way around the new system securities and a new way to "Jtag" your xbox.

Picture of an RGH console:

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You can see the switches on top of the two middle consoles used to switch nands

Now, do not get me wrong, as great as RGH is it has it's flaws. The first, and most noticeable flaw, is boot time. The time it takes for you to turn on and boot up your regular Xbox is around 2-5 seconds, however, with the RGH nand(s) (I'll get into those later) the boot times are extremely inconsistent and can take anywhere from 10-120 seconds! That can get annoying, but usually you can turn it off and then back on and that should make it boot faster. The second flaw is compatibility, this is not nearly as bad as it sounds but not every program that will work on a jtag will work with ggBuild (the program used to make RGH consoles) most have been updated but there are a few that do not work yet. And finally, price, you may have to shell out something around $460 for a dual nand slim alone! However, RGH consoles have plenty of pros too! The first being reliability, when you buy a RGH slim you know every time you turn it on it will work! The second being that with a dual nand you can have a jtag and a retail in one small package! And the final pro being the RGH Dev mod, which I will talk about in the next section about Nand's.
Other pros:
With a slim you have easier access to larger HDD's.
Slim consoles are more power efficient, better looking and quieter.

Nand/HANA chips are basically the police of the xbox 360 motherboard, and the reason you get banned for connecting to xbox live, these chips contain the system codes and Console Hypervisor, which, like it sounds, supervises the console, if it detects anything out of the ordinary it automatically stops the console from connecting to xbox live, and running unlicensed programs. However, with RGH and the CoolRunner NandChip you can boot up your slim/jasper/falcon as a modded console. Although that is nice, the possibilities do not end there, you can add a Cygnos Chip to the console as a second nand (hence dual nand) which allows you to at the flick of a switch go from an online safe retail console, to a modded one, now you must turn it off before switching nands, but the concept is the same. What this allows you to do is have a retail console and a jtag in one console, very useful! The final thing that is extremely impressive is the "RGH Dev" now if you have not heard of a devkit, it is what developers use to test and debug their games, however with the release of RGH, Slim consoles are now able to, with the right software, be turned into a devkit, with access to neighborhood and all, the only differences between the RGH Dev and a regular Devkit is access to Pnet and DVD emulation! This means access to memory in real time (Real Time Editing) screenshots ect. With the release of this program came the Xbox 360 Xperts Prototype Tri-Nand all in one Dev, Retail and "Jtag" console. In simpler terms, it has everything a RGH, Retail, and Dev console has (minus Pnet and DVD emulation). Pretty sweet, anyways, onto the differences between Jtags and RGH Consoles.

The Chips:

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What are the major differences between RGH's and Jtag's? That you notice? Next to none, ohter than the "compatibility issue" which is minor, in essence everything you can do an a jtag you can do on an RGH, however this does not work in reverse, ie: you cannot the RGH dev on a Jtag. That is really just about it, if you have anything to add please feel free to post it and I will add it to the section!