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HyperText Markup Language (HTML)

HTML is written in the form of HTML elements consisting of tags enclosed in angle brackets (like ), within the web page content. HTML tags most commonly come in pairs like 

 and 

The purpose of a web browser is to read HTML documents and compose them into visible or audible web pages. The browser does not display the HTML tags, but uses the tags to interpret the content of the page.

HTML elements form the building blocks of all websites. HTML allows images and objects to be embedded and can be used to create interactive forms. It provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists, links, quotes and other items. It can embed scripts in languages such as JavaScript which affect the behavior of HTML webpages.

Web browsers can also refer to Cascading Style Sheets (CSS) to define the appearance and layout of text and other material. The W3C, maintainer of both the HTML and the CSS standards, encourages the use of CSS over explicitly presentational HTML markup.^[1]

Internet Communication

E-mail

Email, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the same time, in common with instant messaging. Today's email systems are based on a store-and-forward model. Email servers accept, forward, deliver and store messages. Neither the users nor their computers are required to be online simultaneously; they need connect only briefly, typically to an email server, for as long as it takes to send or receive messages.

An email message consists of three components, the message envelope, the message header, and the message body. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually descriptive information is also added, such as a subject header field and a message submission date/time stamp. It is structured into fields such as From, To, CC, Subject, Date, and other information about the email. Body represent the basic content, as unstructured text; sometimes containing a signature block at the end. This is exactly the same as the body of a regular letter.

The header is separated from the body by a blank line.

In the process of transporting email messages between systems, SMTP communicates delivery parameters using a message envelope separate from the message (header and body) itself.

E-mail address

It is not clear from the email address domain name what is the actual destination (the mailbox host) of an email. A mail server will use the Domain Name System, which is a distributed database, to find the IP address of the host of the domain. The server queries the DNS for any mail exchanger records (MX records) to find the IP address of a designated mail transfer agent (MTA) for that address. That way, the organization holding the delegation for a given domain —the mailbox provider— can define which are the target hosts for all email destined to its domain. The mail exchanger does not need to be located in the domain of the destination mail box, it must simply accept mail for the domain. The target hosts are configured with a mechanism to deliver mail to all destination mail boxes. If no MX servers are configured, a mail server queries the A record for the domain. There is a chance that this server will accept email for this domain.

The local-part of an email address has no significance to intermediate mail relay systems other than the final mailbox host. For example, it must not be assumed to be case-insensitive. The same mailbox can be set up to receive emails from multiple email addresses. Conversely, a single email address may be an alias and have a distribution function to many mailboxes. Email aliases, electronic mailing lists, sub-addressing, and catch-all addresses, the latter being mailboxes that receive messages irrespectively of the local part, are common patterns for achieving such results.

The addresses found in the header fields of an email message are not the ones used by SMTP servers to deliver the message. Servers use the so-called message envelope to route mail. While envelope and header addresses may be equal, forged email addresses are often seen in spam, phishing, and many other internet-based scams. This has led to several initiatives which aim to make such forgeries easier to spot.

Spamming and computer viruses

The usefulness of email is being threatened by four phenomena: email bombardment, spamming, phishing, and email worms.

Email spam, also known as junk email or unsolicited bulk email (UBE), is a subset of electronic spam involving nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk.^{[1][2][3][4][5]} One subset of UBE is UCE (unsolicited commercial email). The opposite of "spam", email which one wants, is called "ham", usually when referring to a message's automated analysis (such as Bayesian filtering).^[6] 

Email spam has steadily grown since the early 1990s. Botnets, networks of virus-infected computers, are used to send about 80% of spam. Since the expense of the spam is borne mostly by the recipient,^[7] it is effectively postage due advertising.

Spammers collect email addresses from chatrooms, websites, customer lists, newsgroups, and viruses which harvest users' address books, and are sold to other spammers. They also use a practice known as "email appending" or "epending" in which they use known information about their target (such as a postal address) to search for the target's email address. Much of spam is sent to invalid email addresses.

Email worms use email as a way of replicating themselves into vulnerable computers. Although the first email worm affected UNIX computers, the problem is most common today on the more popular Microsoft Windows operating system.

The combination of spam and worm programs results in users receiving a constant drizzle of junk email, which reduces the usefulness of email as a practical tool.

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