The IBM 3270 is a class of terminals made by IBM since 1972 (known as "display devices") normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text color on the original models, these terminals are informally known as green screen terminals. Unlike common serial ASCII terminals, the 3270 minimizes the number of I/O interrupts required by accepting large blocks of data known as datastreams, and uses a high speed proprietary communications interface, using coax cable.
IBM stopped manufacturing terminals many years ago, but the IBM 3270 protocol is still commonly used via terminal emulation to access some mainframe-based applications. Accordingly, such applications are sometimes referred to as green screen applications. Use of 3270 is slowly diminishing over time as more and more mainframe applications acquire Web interfaces, but some web applications use the technique of "screen scraping" to capture old screens and transfer the data to modern front-ends. Today, many sites such as call centers still find the "green screen" 3270 interface to be more productive and efficient than spending resources to replace them with more modern systems.
In a datastream, both text and control (or formatting functions) are interspersed allowing an entire screen to be "painted" as a single output operation. The concept of "formatting" in these devices allows the screen to be divided into clusters of contiguous character cells for which numerous "attributes" (colour, highlighting, character set, protection from modification) can be set. An attribute occupied a physical location on the screen which also determined the beginning and end of a "field" (separately addressable sub section of the screen).
Further, using a technique known as "read modified," the changes from any number of formatted fields that have been modified can be read as a single input without transferring any other data, another technique to enhance the terminal throughput of the CPU. Some users familiar with character interrupt-driven terminal interfaces (such as Microsoft Windows) find this technique unusual. There was also a "read buffer" capability which transferred the entire content of the 3270-screen buffer including field attributes. This was mainly used for debugging purposes to preserve the application program screen contents while replacing it, temporarily, with debugging information.
The first 3270s had no function keys. Later 3270s had twelve, and later twenty-four, special programmed function keys, or PF keys, and three PA (or "program attention") keys placed in one or two rows at the top of the keyboard. When one of these keys is pressed, it will cause its control unit (historically, usually, an IBM 3274 or 3174, but nowadays the onboard mainframe equivalent) to generate an I/O interrupt and present a special code identifying which key was pressed. Application program functions such as termination, page-up, page-down, or help can be invoked by a single key press, thereby reducing the load on very busy processors.
In this way, the CPU is not interrupted at every keystroke, a scheme which allowed an early 3033 mainframe with only 16 MB to support up to 17,500 3270 terminals under CICS. On the other hand, vi-like behaviour was not possible. (But end-user responsiveness was arguably more predictable with 3270, something users appreciated.) For the same reason, a porting of Lotus 1-2-3 to mainframes with 3279 screens did not meet success because its programmers were not able to properly adapt the spreadsheet's user interface to a "screen at a time" rather than "character at a time" device.
In contrast, IBM's OfficeVision office productivity software enjoyed great success with 3270 interaction because of its design understanding. And for many years the PROFS calendar was the most commonly displayed screen on office terminals around the world.
In contrast also, ICI Mond Division's Works Records System, the first known shared public spreadsheet used the 3270 successfully for what was, in effect, a high powered version of today's spreadsheets with additional functions. It remained in continual use for 27 years up until 2001 and, despite its lack of a GUI, cells could be defined anywhere on the screen (not necessarily in rows or columns) and could be instantly re-configured for length, content and formulas as required. It is interesting to note that ICI's online, fully interactive system pre-dated PC spreadsheets by quite a few years and allowed multiple users to use the spreadsheets at the same time, similar to today's Web based shared spreadsheets such as Editgrid, Google Spreadsheets, and others.
As mentioned above, the Web (and HTTP) is similar to 3270 interaction because the terminal (browser) is given more responsibility for managing presentation and user input, minimizing host interaction while still facilitating server-based information retrieval and processing.
Applications development has in many ways returned to the 3270 approach. In the 3270 era, all application functionality was provided centrally. With the advent of the PC, the idea was to invoke central systems only when absolutely unavoidable, and to do all application processing with local software on the personal computer. Now in the Web era (and with Wikis in particular), the application again is strongly centrally controlled, with only technical functionality distributed to the PC.
In the early 1990's a popular solution to link PCs with the mainframes was the IRMA card. It was a piece of hardware that plugged into a PC and connected to a coaxial cable towards the mainframe. IRMA also allowed file transfers between the PC and the mainframe.
Many manufacturers created 3270 compatible terminals such as Hewlett Packard, or adapted ASCII terminals such as the HP 2640 series to have a similar block-mode capability which would transmit a screen at a time, with some form validation capability. Modern applications are sometimes built upon legacy 3270 applications, using software utilities to capture (screen scraping) screens and transfer the data to web pages or GUI interfaces.
(Generally, 3277 models were upper-case only, except for the mixed EBCDIC/APL which had lower case; lower-case capability and possibility of dead keys, at first a simple RPQ (Request Price Quotation, tailored on request at extra cost) was only added in 3278 & 3279 models.)
Telnet 3270, or TN3270 describes either the process of sending and receiving 3270 data streams using the  protocol or the software that emulates a 3270 class terminal which communicates using that process. TN3270 allows a 3270 terminal emulator to communicate over a TCP/IP network instead of an SNA network. Standard  clients cannot be used as a substitute for TN3270 clients, as they use fundamentally different techniques for exchanging data.