Wh@ is it for?
The identity, of course, is your internet identity. Your email address is an important part of it. The '@' sign, in turn, is an important part of your email address. It separates the user name from its host, as in tom_is(user name)@om.org(host).
The host name signifies the machine at which the user can be found, at least virtually. It is more specific than the domain name. The domain comprises an entire company, for example, with a huge number of computers. The computers are the individual hosts connected via an internal network.
Technically, a domain name is nothing else than an IP address (something like "192.168.0.15") in a more legible form. Domain name and host name can be the same and with email addresses typically are.
The user name typically is the name you use the log in.
'@' is short for "at". This is why it was chosen for the purpose of identifying a person "at" a computer.
'@' should also be pronounced "at". Consequently, our firstname.lastname@example.org becomes "TomIsAtomOrg" It's up to you whether you prefer Tom to be a Tom, an atom or in a meditative "om" state.
Wh@ a history!
In the Middle Ages, monks were what is today the internet. They published, reproduced, kept and passed on vast amounts of data.
The monks' translations and transcriptions were hard work. Hard-working monks had to deal with a unique set of problems. Bookbinders used to join together the wrong pages, for example. To prevent this, scribes and authors repeated the last line on the new page.
Texts were long and brevity a virtue. Where it did not exist, it was created. Authors abbreviated words as short — and common — as "ad". "ad" is Latin for "at".
'@' seems like a logical abbreviation if you know that medieval fonts had a 'd' character that looked much like a mirrored 6.
From Monks to Merchants
In the 15th century, the '@' sign appeared again. Spanish merchants used it — as an abbreviation. The weight measure "arroba" (about 11.52 kg or 25.40 lb.) was soon replaced by the more handy '@'. This measurement was used for bulls and wine, by the way.
During the Renaissance period, people started to use '@' in the more general sense of "costs": "1 email address @ $100" means an email address costs USD 100.
From Merchants to Mail
With the advent of the Industrial Revolution, '@' was used in bookkeeping and that's why the inventors of email happily found it on their keyboards.
Supporting the theory that one form of communication never replaces but only compliments another, a special code for the '@' character was introduced to the Morse code in 2004.