'G' stands for generation, so the higer number means the newer generation or product. In this case, 3G and 4G are new kinds of mobile phone technology. 3G brought broadband access to mobile phone users; the main difference with 4G is that it's faster. However, this is only true if the technology is working on its own carrier - when using other carriers there are often complications.

You can get more data with 4G, so it's good if you use the internet a lot, but can also get very expensive very quickly. However, being the newer technology it is expected to keep improving, and quite soon a lot of new phones will only be suitable for 4G anyway. On the other hand, the iPhone works with 4G and there seem to be no plans to change that.

You can get a good overview of the two products, and find out which is faster for your area, here.