Genres
Western Stories
The earliest dime novels were frontier stories written in the mold of James Fenimore Cooper's Leatherstockings Tales, typically featuring a scout or backwoodsman who guides a party through a dense forest to a fort in 18th century New England. Protagonists would often come into conflict with Native Americans, which would become a recurring feature of the genre and lend it a reputation for racism and jingoism. 
These stories are occasionally fictional accounts of real historical figures, such as Davy Crockett and Daniel Boone, but more often the characters are completely fictional, like Nick Whiffles. It wasn't long before the setting of these novels shifted to the contemporary Western frontier, especially mining camps in the Dakotas and California. At first, outlaws, gamblers, and other anti-heroes were especially popular, such as Edward L. Wheeler's Deadwood Dick or real-life gang leader Jesse James. These characters were almost always driven to a life of crime by their circumstances, but often had basically good intentions. With growing concerns about the moral appropriateness of dime novels for children, however, stories about cowboys and plainsmen came to replace outlaws. The most famous of these was undoubtedly Buffalo Bill, who first appeared in several stories by Ned Buntline for Street and Smith's New York Weekly, then later in dozens of stories by Col. Prentiss Ingraham in the pages of Beadle's New York Dime Library. Because the Western was one of the earliest dime novel genres, many have erroneously assumed that all dime novels are Westerns. In fact, detective fiction would surpass the Western in popularity among contemporary readers.
Detective and Mystery Stories
Most dime novel detective stories follow the same basic formula: the detective is presented with a mystery, usually by a client or a law enforcement officer,
then makes a series of startling discoveries and daring escapes before solving the mystery in a grand denouement. Unlike the detective fiction of Poe or Doyle, there is little analytical deduction. As noted by J. Randolph Cox, much of this fiction followed the pattern set by stories about the real life Pinkerton Detective Agency and the works of French novelist Emile Gaboriau, with the detective shadowing suspects, often in disguise, and conducting exhaustive investigation into their family background. Detective fiction appeared as far back as the 1840s, many of which were set in the West, sometimes even featuring the same characters that appeared in Western fiction. For example, Jesse James would square off against the famous detective Old King Brady on in a few seperate adventures. By the late 1880s, the detective and mystery story had become the dominant genre and would remain so until the format's demise. The genre would ultimately be defined by its heroes, the first of which was Harlan Page Halsey's Old Seuth in 1871. The most popular detectives, each of which had hundreds of adventures and multiple incarnations, were Old King Brady and Nick Carter.
Love Stories
Love stories often focus on courtship leading to marriage or the resolution of some crisis that a couple must face. There is typically a villain who is conspiring to keep the couple apart, often a romantic rival or a family member, which provides the central conflict. 
The plot consists of a series of misunderstandings and seperations, ultimately leading to what Mary Noel calls the "grand reuinion," where secrets are revealed and the heroine is reunited with her true love. Unlike earlier sentimental fiction, in which the threat to the protagonist was mostly psychological, the dime novel romance heroine is usually in some sort of physical danger, especially danger to her virginity. Many of these stories are told from the point of view of the heroine herself, who might be an orphan, an heiress, or a working girl. Almost every dime novel features a love story of some sort, although romance was also a distinct genre. Romance novels would usually be serialized first, very often pirated from English story papers, then published complete in a series. Several series were even devoted exclusively to romance fiction, such as the Waverley Library and the Eagle Library. Unlike many of the other genres described here, there were very few romance heroines, likely because there were few stories to tell after the reunion. One exception is Marion Marlowe, who starred in the series, My Queen.
Historical Fiction
At the dawn of the dime novel era, historical fiction was almost as popular as Western stories, with tales about the American Revolutionary War and the War of 1812 predominating. In many ways, America's popular conception
of its early history can be attributed to these novels, which often focused on the exploits of self-reliant patriots, either spies or privateers, who would heroically stand up against English and Tory oppression. The Civil War was also a popular source of stories, even though few dime novels about that conflict appeared during the war itself. Two of the first series to focus on the Civil War, the War Library and Five Cent Weekly Library, appeared two decades after the conflict ended and told stories almost exclusively from the Union's perspective. Later, in 1904, the Blue and Gray Weekly would alternative perspective between its Union and Confederate protagonists every other issue. Real historical figures sometimes featured in these novels, like George Washington or Ulysses S. Grant, but more often these stories deal with few real historical events or characters, even sometimes leaving the date unspecified (e.g. 187-). When historical figures do appear, their character is often based on their popular conception, with seldom any relation to reality.
Science Fiction
The 19th century was a period of rapid technological progress, which is reflected in the many stories about boy inventors, also referred to as "Edisonades." In most of these novels, a boy invents a fantastic new mode of transportation, which enables him to go on adventures in far-off lands, where he brings Western civilization to the indigenous people he encounters. 
In other cases, the boy hero uses his inventions to defend his country against foreign invaders. The "yellow peril" stereotype can be traced back to Tom Edison, Jr.'s Electric Sea Spider; or, The Wizard of the Submarine World. In many ways, these novels were not very different from the Western stories, especially in regards to the jingoism and racism that often predominated. The earliest of these, Edward S. Ellis' The Huge Hunter, or the Steam Man of the Prairies, is about a wagon drawn by a steam-powered robot, which was built and operated by the dwarf, Johnny Brainerd. Brainerd was followed ten years later by Frank Reade and his son, Frank Reade, Jr, who were by far the most popular inventors in the genre, with over 200 stories published about their exploits between 1876 and 1899. Other popular characters include Jack Wright and Tom Edison, Jr.
Also popular was the lost race novel, which often overlapped with the boy inventor stories. In lost city novels, the male protagonist discovers a hidden civilization, often full of treasures for him to loot. The most popular lost races were Old Norse and Pre-Columbian American peoples, especially Aztecs. Sometimes these races are far more technologically and socially advanced, as is the case in most Nick Carter lost race stories, but just as often the races are presented as being inferior.
Sea Stories
Sea stories encompass a wide range of narratives and, unlike most of the other dime novel genres, has no single formula. This includes virtually any story set at sea, such as tales of whaling, pirates, shipwrecks, castaways, treasure hunts, and South Sea Island romances. There were few series wholly devoted to the genre, although most anthology series contain a great deal of sea fiction, particularly Fame and Fortune Weekly and Pluck and Luck. In fact, many of the earliest dime novels were sea stories.
School and Sport Stories
In the school and sport story, the male protagonist must clear his name or escape captivity in order to win the big game for his boarding school, military academy, or college. The genre has its roots in British fiction for boys, beginning with Thomas Hughes' Tom Brown's School Days. By far the most popular schoolboy heroes were Frank Merriwell and Fred Fearnot.
Humorous stories
The vast majority of what passed for humor in dime novels is based on exaggerated racial and ethnic stereotypes, typically featuring characters who speak in dialect. Many of these characters appear in Western stories, mystery and detective stories, and science fiction, often relied on for comedic relief. Frank Tousey also published a number of comic stories for his Five Cent Wide Awake Library and Five Cent Comic Library, the most popular of which were about the Irish immigrant Terence Muldoon and a family of dwarf entertainers, the Shortys.