It's basically the theft of your credentials (user name and password, credit card number, bank account numbers, etc.) by people who then sell that information to other parties. It's then used for other more serious crimes like robbing banks, identity theft, international terrorism, and good old fashioned fraud.
While most phishing is aimed at banks and other financial institutions, we have seen some aimed at Gandi customers. This might seem odd, since Gandi has all low-value transactions. We also don't store any credit card data, paypal logins, or anything really intrinsically valuable that would help a hacker access your account at a financial institution. Why then try to steal your Gandi account?
The answer lies in what you have already bought at Gandi that does have intrinsic value: your domain name.
Phishers want to impersonate banks and financial entities, and they don't want to be caught. If they register a domain for this, Gandi generally shuts them down pretty fast, and the contact data and other information they enter can potentially be used to track them down. It's less useful than a legitimate domain with no visible connection to them, that is... your domain.
Recent trends in phishing attacks tend to bear this out. The first half of 2012 saw a large increase in the use of hacked domains for phishing attacks, and a decrease in the overall number of domains registered for phishing purposes. The phishers are smart, and they are after your Gandi account. They want it so that they can use your legitimate domains for no good purpose.
Everyone should use caution: use a strong password. Never share your password with anyone. Use different passwords for your email, Gandi handle, and online bank accounts, etc. Never click on links or attachments in email unless you are sure of where they come from and where they go. Don't be a Phish!