Question: What is the difference between 100 Mbit/s and 1 Gbit/s? What are the advantages of a 1 Gbit/s network and pricing?

Answer: 
**It's probably the line rate (speed) of the first network link, the one which attaches your server to the hosting providers network.** It matches the ubiquitous 100 Mbit/s and 1 Gbit/s Ethernet standards; and hosting providers have a habit of mentioning this link speed, as it gives such a nice high number to show.

The actual difference is between 100 megabit/second (which often comes to around 11 megabyte/s in practice), and 1000 megabit/s (often around 80 megabyte/s).

In most cases, **it is a pretty meaningless measurement on its own.** Large-scale Internet traffic is expensive, so no hosting provider will really sell you 100 or 1000 Mbit/s **out of their network** for a few hundred bucks per month. In other words, you will be limited elsewhere -- limited by the practical speed the hosting providers own network can handle, or by [traffic shaping](http://en.wikipedia.org/wiki/Traffic_shaping), or by an "acceptable use policy" or other legal agreement.

**Edit:** I should add that if your site can work on a single server, then you most likely don't need more than a few megabit per second. Earlier on the Stack Overflow blog had a [nice post with numbers and bandwidth usage](http://blog.stackoverflow.com/2009/02/happy-100000th-question/), showing Stack Overflow consuming only around 6 Mbit/s. Now, Stack Overflow is a best-case example, as it is a site with mostly text, HTTP compression turned on, and a proper HTTP headers set. So analyze your own needs; [YMMV](http://en.wiktionary.org/wiki/YMMV).
1 gigabit/s network is ten times faster, and therefore more capable, than 100 megabit/s - so 1 Gbit/s should be better than 100 Mbit/s - that said, if your server doesn't use this extra capacity, it may not be worth going for the 1 Gbit/s. What's the price difference?