Differences Between Wireless N and G Protocols

802.11 Wireless Protocol Series

802.11 is the wireless protocol series of standardization created by the IEEE (Institute of Electrical and Electronic Engineers) society. 802 is the number classifying all standards relating to Networking, be it WAN, LAN, or WLAN technologies. 802.11 is specific to standards dealing with wireless networking. That standard is further broken down into letters, such as 802.11a, 802.11b, 802.11g, etc.
As you can see, there are some letters missing in there. When these standards were initially proposed each proposition was given a letter, and a team was assigned to develop them. It was never intended for every one of them to be a success, and many didn't make it past the theoretical stages of development.
The sole purpose of this standardization process is so that you and I can go the store, purchase a wireless router from Linksys and a wireless card for our laptop from Dlink, and know that the two will work together, so long as they both support the same standard. Without these standardizations of wireless protocols, there would be dozens of proprietary standards out there and nothing would work together. There would be no coffee shop LANs or free access at the library - it would all be a mess.

Today's Most Common Wireless Standards

While 802.11a and 802.11b were used for quite some time, today the vast majority of wireless devices run on either the older 802.11g protocol or the newer 802.11n protocol. There are many differences between these two standards, meaning they are not "backwards compatible" like 802.11g was to 802.11b. However, if you are using a mix of N and G standardized devices, most wireless routers that support N will also support G - but count on speeds being limited to the "oldest" standard currently running.

Features of 802.11g

802.11g was the fastest wireless networking protocol in existence until wireless N drew closer to completion. Like 802.11a, 802.11g supports a maximum speed of 56Mbps (mega-bits per second). However, unlike 802.11a it functions on the 2.4GHz band. This means that G was not compatible with A. However, the slower (11Mbps) 802.11b was compatible, as it also worked on the 2.4GHz band.
The problem with G is not so much that it is too slow (56Mbps for wireless is pretty respectable) but that it shares a wireless band with many common devices that cause considerable interference. For example, most cell phones, cordless phones, Bluetooth, and microwaves share the 2.4GHz band. In fact, if it were not for this problem we probably would not have an 802.11n in the first place, since 802.11g can be pushed up to 108Mbps on some devices ("super-G").

So, to recap:
Standard: 802.11g
Band: 2.4GHz
Speed: 54Mbps, or 108 Mbps

Features of 802.11n

802.11n is an ambitious standard. In fact, it is so ambitious that the proposed standard has been revised multiple times, and has been "on the table" so to speak for nearly 5 years. It is worth noting that the 802.11n devices that we see in stores now are based upon a proposed 802.11n standard from 2007 - it hasn't been accepted or finalized by the IEEE.
Wireless N uses a technology known in the industry as MIMO. This stands for multiple input, multiple output. In effect, instead of a device having a single antenna, it has multiple antennae that are arranged so as to have maximum transmitting power, and maximum reception. In simple terms, it is the difference between trying to shoot a fish with an arrow and simply grabbing it with a net.
Wireless N claims "real world" speeds of 130-160Mbps, with a listed maximum of 300Mbps. This is reliant on the router transmitting in full 802.11n mode, which is impossible when an 802.11g device is also connected to the network.
As far as the wireless band goes, 802.11n devices can operate on 2.4 or 5GHz bands. However, to use 5GHz all connected devices must be 802.11n compliant. Note, too, the 5GHz band will significantly decrease the maximum range. The tradeoff is that there will be much less interference, and 5GHz is drastically more effective at penetrating concrete and brick walls.