All online usage involves the downloading of information from the internet. GB (gigabytes) indicates the amount of data – like web pages, songs, albums and movies – you can download/upload each month.
40GB is perfect for regular internet usage – email, Facebook, browsing and downloading larger files occassionally (about 40 movies per month for example).
300GB is designed for heavier users. If you download large files and movies or game online more regularly, 300GB is the usage allowance you'll need (it allows, for example, about 300 movies per month).
350GB is for really heavy users. If you're perpetually streaming video or are a mad online gamer, for example, this would be the plan for you.
eircom claims to give "unlimited" usage with certain plans. But look out for the terms and conditions – eircom's fair usage cap is 250GB. You'll always get more with Vodafone.