SMS cards were originally created for the IBM 7030 Stretch supercomputer. The idea of SMS cards was to have a few standard cards that formed the building blocks that could be combined to build a computer. Each card provides a basic function such as a flip flop or a few logic gates. The board below, for example, implements three AND gates, using a relatively slow type of logic called diode-transistor logic that only requires one transistor per gate. You can see the transistors (silver circles) and diodes on the right, while resistors and inductors are on the left.
SMS cards became the basis of of IBM's systems and were used on computers such as the IBM 1401, IBM 1620, and IBM 7000 series, as well as in tape drivers, printers, and other peripheral devices. SMS cards continued to be used well into the 1970s, even after integrated circuits made discrete transistors obsolete: advanced mainframes of the 1970s such as the IBM 370 still used multiple SMS cards for power supply regulation.
The picture below shows the back of an SMS card. The 16 gold-plated contacts on the end plug into a socket in the computer, connecting the card. The circuit board pattern is considerably more complex than necessary; depending on the components installed, one circuit board can implement several different SMS cards, reducing manufacturing costs.
Since each SMS card is so basic, it takes thousands of them to build a computer. The picture below shows an IBM 1401 mainframe, a very popular business computer of the 1960s. One of the racks of cards (which IBM confusingly calls a "gate") is open, showing more than 125 SMS cards plugged in. The circuitry in this specific gate below implements timing and logic functions (such as addition and subtraction). For maintenance, each gate swings out of the computer simply by pulling on the handle above the fan. The 1401 has 24 gates like this full of SMS cards, each one implementing different functionality of the computer. In total, the IBM 1401 computer contains more than 3400 SMS cards.
The picture below is a closeup of the SMS cards plugged into a gate in the IBM 1401. At the top of the gate, wiring harnesses connect this circuitry to other parts of the computer. On the back of each gate, the SMS cards are connected together by wirewrapping.
Although the original idea of SMS cards was to standardize on a few types, the number of different cards exploded as time went one, resulting in thousands of different SMS card types. As well as logic gates, SMS cards can have an amazing variety of functions such as an oscillator, voltage regulator, core memory, fuses, printer hammer driver, disk speed detector or temperature switch. The 1401 computer alone uses 162 different types of SMS cards.
Information on particular SMS cards is surprisingly hard to find, so I made a database of SMS cards, collecting information on 900 different cards. Given the historical importance of SMS cards, I think information on this technology should be preserved. I pulled together information from scans of old IBM documents and a bunch of other sources (which was more work than I expected). You can access the database at righto.com/sms and see the wide variety of cards along with photos, descriptions, schematics, and the devices that use them.
In the 1960s, IBM made SMS cards by the millions, but most of them have been scrapped for the gold in their contacts. You can still find a few SMS cards on places such as eBay, though, where they are collectables for about $15 each. If you want to see SMS cards in operation, the Computer History Museum in Mountain View has live demonstrations of the 1401 computer on Wednesdays and Saturdays (schedule). Check it out if you're in the area.
In conclusion, SMS cards are an important part of computer history of the 1960s. It's hard to imagine computers before silicon, but SMS cards provide a window into that time. While the technology of today's computers is hidden away in microchips, the whole circuitry of older computers is exposed, letting you see how individual components make simple logic circuits, these boards are combined into functional units, and the computer is built from these units. And if technology had progressed slightly differently, this area might be known as Germanium Valley instead of Silicon Valley.
Notes and references For the first few years, transistors were made of germanium, not silicon. In 1954, silicon transistors were introduced and rapidly took over because they were more stable and had better operating characteristics. Germanium transistors are almost entirely obsolete now, but they still remain popular in fuzz and distortion pedals, where they are said to give a better sound than silicon transistors.
 The IBM 7030 Stretch supercomputer is described in detail in The engineering design of the Stretch Computer, 1959. The Stretch used 22,000 SMS cards, of which 4,000 were double cards. Stretch used just 42 different types of SMS cards (24 single-width and 18 double-width), with two types of cards making up more than half of the computer. In comparison, the IBM 1401 is a much smaller system with just 3047 total cards, but it has more than 120 different card types. Some cards can be modified by jumpers; counting the variants the 1401 has 162 different card types. (The 3047 figure is for the Computer History Museum's IBM 1401; the figure will vary for models with different optional features.)
 The JGVW SMS card has three two-input AND gates, running on -12V. Inputs are +6V or -6V, outputs are -12V or 0V. (SMS cards use a variety of voltages for their inputs and outputs.) Details on this SMS card are here.
 You don't see inductors in logic circuits very often these days, but inductors were commonly used on SMS cards to improve performance. The inductors filtered the output signals to make the signal transitions faster.
 To figure out the value of the gold in an SMS card, I measured the gold contacts as 2.26mm by 10.74mm with 1.62mm by 1.13 mm traces, which works out to 24.27mm^2 per contact. With 16 contacts and a thickness of 2.54 microns, that works out to just over 1 cubic mm of gold per SMS card. At the current gold price of $1200 per ounce, that's 79 cents of gold per board. In total, the 3047 SMS cards in an IBM 1401 contain $2400 worth of gold.