Bandwidth in general means the amount of data that can be transmitted from a communication channel within a fixed amount of time. In Web Hosting, Bandwidth is the amount of information transferred (downloaded/uploaded) from the web server over a specified period of time. Even though bandwidth capacity is measured by transfer rate per second, in Web Hosting industry it is generally considered as total allotted transfer amount by hosting provider to their customer in given month.
A web hosting plan that offers 100 GB/month bandwidth, could also mean, that your web site will be taken offline/suspended till the end of the month by your web hosting service provider if your account exceeds 100 GB Bandwidth. However there are many web hosts who instead of suspending hosting account, will charge your account for additional used bandwidth as previously agreed with customers.
1 kbit/s (one kilobit or one thousand bits per second) = 1,000 bit/s
1 Mbit/s (one megabit or one million bits per second) = 1,000,000 bit/s
1 Gbit/s (one gigabit or one billion bits per second) = 1,000,000,000 bit/s