When you open a twitter account, you have to give them an email address. But it has to be an email address that does not already have a twitter account. So if you want to open multiple twitter accounts (one for friends, one for work, one for certain hobbies, etc), you need a unique email address for each account. If you don't want to use multiple email addresses (or you don't have multiple email addresses), you can trick twitter into letting you use multiple accounts with a single email address.

Many email service providers (GMail is one of them) ignore periods in the name part of the email address. So if you have an email address like myname@gmail.com, you can actually get emails delivered to this address if people put a . anywhere in the myname section. For example, my.name, m.yname, mynam.e, etc. So find out if your email provider works this way. If not, get a Gmail account. You can then sign up for multiple twitter accounts and just add periods to the addresses to make twitter think they are unique.