Considering the number of three-quarters of Facebook users use Gmail (no connection between the services intended), it's one of, if not the most popular web email service there is on the web. It's clear to me already that students use Gmail more than any other service available, and often use it to outsource their own university email.
There is no reason as of yet as to why Gmail failed this afternoon, although the service does seem to be up and running once again; according to their status page:
Update at 1:30 UTC: Please note that the Google Mail service has already been restored for the majority of our users. The rest of the users should be coming back online within the next 24-36 hours. We apologize for the inconvenience and thank you for your patience and continued support.
Gmail, to me, remains as one of the most important and integral parts of my life at the moment. Without the service, I wouldn't be able to get the email on my phone, access my email from everywhere with any browser, and chat with my friends and colleagues all in the same window.