Bandwidth is the measure of maximum data that can be transferred in a given time, usually measured in seconds.
Bandwidth should not be confused with data transfer as they are two different things. Data transfer is the amount of data to be transferred; while bandwidth is the rate of the data transfer.
You can imagine bandwidth as the width of a water pipe where data transfer is the amount of water flowing out from the pipe. How wide is the pipe width (bandwidth) determines how fast can water (data) flows. Fundamentally, data transfer is the consumption of bandwidth.
How bandwidth affects your site?
The lower the bandwidth, the slower your site loads, regardless on your visitors Internet connections. When your website is heavily trafficked, some of your web users will even need to wait for their turn to access your website.
Is unlimited bandwidth for real?
To win market segment in fierce competition, some hosting companies are now offering unlimited bandwidth, which is impossible in a practical world.
The logic behind unlimited bandwidth is that hosting companies are banking on the fact that most webmasters will only consume a very small amount of bandwidth. Thus via strategic planning and careful hardware monitoring, hosting companies are able to offer a bandwidth capacity that can’t be dried up virtually. You can read more about marketing trick with unlimited bandwidth offer here.
How to reduce bandwidth usage?
Also, out of control robots can also suck down your bandwidth like a black hole. Avoid unknown bots crawling your website via a filter list on robots.txt also helps a lot in reducing bandwidth usage.