Naturally, you should read samples of previous issues for periodicals, but for anthologies or new publishers, you may not have that luxury. If you have read more than a few books in the genre you are trying to write for, you may already understand the concepts but not know the names. For example, I’ll describe the aforementioned genres.
Hardboiled refers to the subgenre of crime fiction that makes use of a detached voice that is indifferent about the sex and violence encountered in the story. A good example would be Mickey Spillane’s Mike Hammer, which followed in the same vein as Philip Marlowe and Sam Spade.
Steampunk is generally science fiction or fantasy that takes place in the Victorian age of Steam often with mechanical devices that did not exist at the time. The background culture is often though not always dystopian. A good example would be the writings of HG Wells or Jules Verne.
Regency is a subgenre of romance focused around the era of the early 19th Century. Publishers would expect both historical accuracy and a reserved tone, witty rather than racy. Look to Jane Austen for help in this subgenre.
Of course, you should do your own research. I am just giving you a general idea. Remember, good readers make good writers. Good luck and happy writing!