The term "Fantasy of Manners" was first coined by Ellen Kushner, the author of the Swordspoint series, and one of the modern pioneers of the genre. Understandably, the genre often gets confused with Steampunk, since certain elements may overlap. However, Mannerpunk often places greater emphasis on society norms rather than epic adventures or retro-technology (which pure Steampunk is well-known for.) Characters often engage in battles of wit and manners, rather than weapons—except for the occasional gentleman's duel.
An article I read once summed "Mannerpunk" up as "Jane Austen meets Fantasy" which I believe is the easiest way to explain it in a nutshell. Quite a few Mannerpunk works are set in the Regency period of England, but they can take place in yet another time period or even an alternate country or world, as in the case of "Swordspoint." Also, while Austen may be one of the authors that writers in this genre often look to for influence, some others may draw inspiration from other Classic-era writers as well.
Below are some published books which can be classified under this genre:
The Gormenghast series by Mervyn Peake
The Last Unicorn by Peter S. Beagle
Sorcery and Cecilia by Caroline Stevermer
The Stranger at the Wedding by Barbara Hambly
Swordspoint by Ellen Kushner
Pride and Prejudice and Zombies
Jonathan Strange and Mr. Norrell
Sorcerer to the Crown by Zen Cho
The Goblin Emperor by Katherine Addison
Captive Prince by C.S. Pacat
The Parasol Protectorate series by Gail Carriger
The Glamourist Histories series by Mary Robinette Kowal