Bandwidth is how much data can be transferred at a time in other words it the internet connection speed your web host server is connected to the internet.
Data transfer is how much data is being transferred in a period say month as most companies give monthly rates so I used month to help you understand better.
By Example if bandwidth were a bridge, then the bigger the bridge is the more vehicles can pass through it at the same time. While data transfer is the number of vehicles allowed on the bridge in say a month. In essence, data transfer is the consumption of bandwidth.
Question arise how it affects your site? Let me explain this in a simple manner; The less bandwidth you have, the slower your site takes to load regardless of the visitor's connection type. This means that your web host has a slow line that is connected to the internet. If you have more visitors, some of them will have to wait their turn.
Data transfer you have is monthly allotted by most of the web host companies, if you have purchased a bandwidth quota say 1GB/month and you have good traffic coming to your site than you'll find your site unavailable because you're reached the maximum allowed until a new month rolls by or you upgrade your account.
Always look into this matter before choosing a web host and purchase from a reliable hosting company like Netfirms, iPowerWeb and many others, their servers are connected to internet via premium internet lines which provide lightening speed access to your web site and visitors don't have to wait for the page to load.
Wishing you best of luck for your successful online business.
