Just to clarifiy it, "G" stands for generation, and signifies major advancements in mobile technology. It's not marketing fluff. There ARE benefits with each new G errr generation. Sure, implementations vary, but there are benefits.
Too bored to mention them all here, but:
1G: analog audio (=backround noise, people can eavesdrop on your conversation by tuning to your frequency, no text/data services)
2G: Digital (=voice is digitized and tranfered digitally, uses encyption that can't be easily cracked protecting you from casual eavesdroppers. You can send data/text, but they have to be converted to "voice" first, like was happens on a PSTN modem, and then the "voice" is converted to digital. Also, in order to get data/text services on a 2G cellphone, the cellphone "calls" a number that is the carrier's data/text number. Transfer speed around 9Kbits. You are charged according to the duration of the "call")
2.5: Introduction of GPRS (=uses the digital channel the 2G network provides to send data directly, without having to convert them to voice first, which increases speed to 40-45Kbits, and also charges you for the amount of data you exchange and not the duration of the session)
2.75: Introduction of EDGE (= a tuned version of GPRS, speed increased to 128K)
3G: Introduction of UMTS (= speed is boosted to 300Kbps, making video calls and video streaming possible but at only QCIF resolution, plus faster data services).
3.x: Introduction of HSPA (= massive speed boost, where HSPA signal is available. Makes streamng and video conference at SD and even 720p resolution possible, plus very fast data service).
4G: This is where things start to get blurry. Normally, only LTE Advanced cellphone are 4th generation cellphones, and hence can be labeled "4G", but it had been switched to mean just LTE (a 3.9G technology). Plus, manufacturers may call devices "4G" even if they are just 3.75G, not even 3.9G.
Long story short, 1G - 3.9G mean something, 4G not.
G stands for "Generation" and is purely a marketing term.
The actual technology (LTE or WiMax) as well as the maximum theoretical data rate, depends on the vendor. All consumers know is "bigger numbers go faster".
Marketing uses it as a tool to keep you buying new gear every year or two.