Broadband with a download limit of 6gb?

I don't understand what they refer to as downloading is it just music or films or running of a receipt off Ebay? I have a 512k connection speed at the moment with no download limits but would like to upgrade to l mb or 2 mb with a 6gb and 10gb download limit respectively. Or am I best going for a 1mb with no download limits. HELP! Thank you in anticipation for your answers.