Signature block
A signature block (often abbreviated as signature, sig block, sig file, or just sig) is a block of text automatically appended at the bottom of an e-mail message, Usenet article, or forum post. This has the effect of "signing off" the message. A common practice is to have one or more lines containing some brief information on the author of the message.
Information usually contained in a sig block includes the poster's name, phone number and email address, along with other contact details if required, such as URLs for sites owned or favoured by the author. A witty or profound quotation is often included (occasionally automatically generated by such tools as fortune), or an ASCII art picture. Strict rules of capitalization are not followed. Among some groups of people it has been common to include self-classification codes, though the practice is waning.
E-mail and Usenet
Since by definition these blocks are added automatically to a message, usually regardless of its content, there are guidelines of netiquette regarding their size. The most common guideline, called the McQuary limit, is a size of no more than four lines of less than eighty columns each. This keeps the overall size of the message down, conserving bandwidth as well as the time required to read the message, and ensures that eighty-column terminals (the most common terminal width by far) can display the sig block properly, allowing for programs that reserve the last column for a continuation character; using all eighty columns for text can result in a character wrapping to the next line.
The formatting of the sig block is prescribed somewhat more firmly: it should be displayed as plain text in a fixed-width font (no HTML, images, or other rich text), and must be delimited from the body of the message by a single line consisting of exactly two hyphens, followed by a space, followed by the end of line (i.e., "-- \n"). This latter prescription, which goes by many names, including "sig dashes", "signature cut line", and "sig-marker", allows software to automatically mark or remove the sig block as the receiver desires. A correct delimiter is required for a news posting program to receive the Good Netkeeping Seal of Approval.
However, whether due to ignorance or disregard for these guidelines, a great many people use sig blocks that are either formatted improperly or larger than these suggested dimensions. In past decades, such practice was referred to as warlording, named for one particular Usenet regular who openly flouted the guidelines with sig blocks stretching to many hundreds of lines of ASCII art on messages with little or no relevant content.
Many corporations have internal policies requiring outgoing emails to have lengthy "signatures" appended to them, listing dozens of contact methods, disclaiming legal liabilities, notifying of virus scanning methods, and so forth. These corporate signatures are almost universally large (often larger than the message itself), and composed without regard for the netiquette guidelines described above; they are seen as obnoxious and irritating by many who receive them.
The use of top-posting (with fullquoting) compounds the problem by causing messages to have ever-lengthening tails of garbage hanging off their end, as various corporate disclaimers, ads from free mail services, mailing list footers, and the like get added.
In what seems like a way of protesting large sigs, Gmail takes the Signiture block and collapses it leaving a link "- Show quoted text -" which if pressed shows the text again, however makes it really easy to gloss over it and takes it out of the way. Gives you back your screen for the important parts of the message. Makes you wonder if they are actually only storing it once, it could be a really good way of doing compression.
Internet forums
On web forums, the rules are often less strict on how a signature block is formatted, as Web browsers typically are not operated within the same constraints as text interface applications. Users will typically use a "signature" text area in their given profile for input, which can then allow a user to turn off signatures. Depending on the board's capabilities, signatures may range from a simple line or two of text to an elaborately-constructed HTML piece. Images are often allowed as well, including dynamically updated images, usually hosted remotely and modified by a server-side script.
Signatures are seen as an art form by many of their creators, and there are many websites centered around their creation and display. Most of these websites have competitions, battles and a signature of the week contest, where members submit their entries to have them featured on the main page.
Fidonet
In Fidonet these signatures usually include the address of the originator of the message and are called "origins".