They're known as "TAC"s for short, or even "bionic cervix" if you see it mentioned. It's basically for women who are unlikely to have a successful pregnancy with a regular vaginal stitch (TVC) - whether it's because they've already had a failed prophylactic TVC, or their cervix is damaged (like mine, when a TVC tore it) or their cervix is crazy short (also like mine - barely 1cm at 13 weeks)
The success rate with a TAC (ignoring non incompetent-cervix issues) is around 95-97% which considering most people who have one would almost certainly lose a baby with a TVC, are incredible odds. It can be fitted before pregnancy (around half are), or at around 11-14 weeks - I had mine at 13 weeks. It is permanent and never comes out, and can be used for subsequent pregnancies.
It can be fitted laparoscopically, but unless the doctor is very skilled, the success rate isn't as high. So usually it is fitted via an incision in the same spot (and size) as a c-section incision under GA or spinal. The stitch itself goes around the very top of the cervix - it's more of a loop of tape than a stitch, and then tied closed. It essentially replaces the cervix entirely. Recovery is the same as with a c-section, except in some cases you are still pregnant!
It doesn't interfere with periods or sperm, and you can still have a D&C up to 12 weeks if you have a first trimester loss. But after that point, as the stitch is permanent, you have to have a classical c-section for early losses/deliveries, or a regular c-section if you get to term.
Not only are the odds much better than with a TVC, but you are also a lot less likely to need bedrest or close monitoring, and you are less likely to get an infection or have waters break due to funnelling. Most doctors even say you can still have sex. Aside from the other screen positive issues and my SPD, this has been a completely normal pregnancy - a total contrast to my rescue TVC I had last time!
Unfortunately many, in fact most, doctors see them as overkill as it's major surgery, and seriously prefer women to lose a couple of babies first to prove it's needed. Ironically it's those same doctors who yell "c-section" at the drop of a hat so I don't get it. Anyway, it took a lot of digging around before I found consultants that would do it in this country - there are only around half a dozen. My consultant at C&W where I had it done is Mr Wales and he is lovely - he immediately agreed even though I had not lost a baby (had come very, very close though with my last pregnancy!) and given how my cervix looked at 13 weeks this time out - 1cm or so and "mushy" - I am very glad I have a TAC.
So that's probably everything and more that you ever wanted to know about TACs 
Who are you seeing at C&W?