The Domain Name
Domains on the internet follow a hierarchical system. There are a certain number of top-level domains ("com", "org", "info", "de" and other country codes, for example), which build the last part of every domain name. Within such a top-level domain, custom domain names are assigned to people and organizations applying for them. "about" is an example of such a custom domain name. The domain owner can then set up sub-level domains freely, to form something like "boetius.example.com".
Unless you buy your own domain, you don't have much say (or even choice) concerning the right, the domain name part of your email address.
The User Name
To the "left" of the '@' sign is the user name. It designates who at a domain is the owner of an email address, for example "me".
If it was not assigned to you by your school or employer (or friend), you can choose the user name freely. When you sign up for a free email account, for example, you can enter your own creative user name.
You are not completely free, however. In fact, the number of characters that can be used the user name part of email addresses is literally numbered. Everything that is not allowed explicitly is forbidden.
Characters Allowed in Email Addresses
Now, what are the characters that can be used to build an email address? If we consult the relevant internet standard document, RFC 2822, identifying them seems to be a horrendously complicated endeavor.
The user name consists of words, separated by dots [`.']. A word is an "atom" or a quoted string. An "atom" is a sequence of ASCII characters (from 33 to 126; 0 to 31 and 127 are control characters, 32 is whitespace), excluding braces (`(', `)', `[', `]', `<', `>'), punctuation marks (`.', `,', `;', `:'), two other characters (`\', `"'), spaces (` ') and the mighty (`@'). A quoted string begins and ends with a quotation character (`"'). In between the quotes, you can put any ASCII character (now from 0 to 177) excluding the quote itself (`"') and the carriage return (`\r'). You can quote the quote with a backslash (`\') to include it. The backslash will quote any character. The backslash causes the following character to lose the special meaning it usually would have in the context. For example `\"' does not end the quoted string but appears as a quote in it.
I think it's best if we forget all this (quoted or not) quickly.
Characters You Should Use in Your Email Address
What the standard boils down to is using any ASCII alphanumeric character plus some fancy, but otherwise "normal" characters (`!', `#', `$', `% `&', `*' `+', `-' `~', and whatever you can find in between ASCII 33 and 47).
In short, use lower case characters, numbers and the underscore to create your email address.