Gmail is awesome! It gives you 1 Gigabyte of storage space and many ways to organize your mail. The only draw back is that it has targeted ads that show up near your emails (like how they show up on the side when you google something). Gmail however is safe and the ads are only triggered by certain words in your emails, the mails are not read at all. You are very lucky to have a gmail invite as they are scarce. This is their main privacy policy:
We collect limited account information and store and maintain your account and email messages on our secure servers.
We will never rent, sell or share information that personally identifies you for marketing purposes without your express permission.
We serve highly relevant ads and other information as part of the service using our unique content-targeting technology. No human reads your email to target ads or related information to you without your consent.
In other words, you have nothing to fear, go sign up!
You get Gmail by being invited by someone who has invites. It works like a tree..you invite someone, they invite two other people, etc, etc. It is rather hard to get a Gmail account because you can't just sign up for one. Your best bet would be to buy an invite on Ebay or ask around to see if you know someone who has one.