Web site traffic, how many users are in a Gigabyte?
It means 20GB total traffic per period (month?). So, if a typical visitor viewed 20MB, ie lots of pictures, then 1000 of those types of visits could be made in the period. If they only viewed 5MB, then 4 times as many visitors would be OK. Text on web pages is tiny compared to image content. 20GB would be enough for your friends, but not enough for a popular business site.
A gigabyte relates to the amount of information that will be sent to people visiting the site. You would not be limited to the amount of users…. 20Gb traffic per month would be more than enough for a small to medium website, High volume sites like youtube would need 2048Gb (2TB) per hour, I would say you will be fine… You can get hosting with unlimited traffic though if you are concerned.