Hi5 is a website that allows you to create a "friends network", it is similar to Bebo, Friendster, or Ringo.
Some people like the site, but other people are more aware of it's security and privacy issues. When you sign up for Hi5, you are asked to give them your email password, which is a bad idea, so they can import your address book into their database. They then proceed to send out invites, without your knowledge, to everyone on your address book. Many people consider this spam, and annoying.
"hi5 collects personal information when you register, when you use hi5, when you visit hi5 pages or the pages of certain hi5 partners. hi5 may combine information about you that we have with information we obtain from business partners or other companies. Once you register with hi5 and sign in to our services, you are not anonymous to us. hi5 collects information about your transactions with us and with some of our business partners. hi5 automatically receives and records information on our server logs from your browser."
Take what you will from it, but it is advised that you not have anything to do with this site.