The term B movie originally referred to a motion picture made on a low or modest budget and intended for distribution as the less-publicized, bottom half of a double feature during the so-called Golden Age of Hollywood. Although the U.S. production of movies intended as second features largely ceased by the end of the 1950s, the term B movie continues to be used in a broader sense, referring to any low-budget, commercial motion picture meant neither as an arthouse film nor as pornography. In its post–Golden Age usage, there is ambiguity on both sides: on the one hand, many B movies display a high degree of craft and aesthetic ingenuity; on the other, the primary interest of many inexpensive exploitation films is prurient. In some cases, both are true.

In either usage, most B movies represent a particular genre—the Western was a Golden Age B movie staple, while low-budget science-fiction and horror films became more popular in the 1950s. Early B movies were often part of series in which the star repeatedly played the same character. Almost always shorter than the top-billed films they were paired with, many had running times of 70 minutes or less. The term connoted a general perception that B movies were inferior to the more handsomely budgeted headliners; individual B films were often ignored by critics. Latter-day B movies still sometimes inspire multiple sequels, but series are less common. As the average running time of top-of-the-line films increased, so did that of B pictures. In its current usage, the term has two primary and somewhat contradictory connotations: it may signal an opinion that a certain movie is (a) a genre film with minimal artistic ambitions or (b) a lively, energetic film uninhibited by the constraints imposed on more expensive projects and unburdened by the conventions of putatively "serious" independent film. The term is also now used loosely to refer to some higher budgeted, mainstream films with exploitation-style content, usually in genres traditionally associated with the B movie.

From their beginnings to the present day, B movies have provided opportunities both for those coming up in the profession and others whose careers are waning. Celebrated filmmakers such as Anthony Mann and Jonathan Demme learned their craft in B movies. B movies are where actors such as John Wayne and Jack Nicholson became established, and the Bs have also provided work for former A movie actors, such as Vincent Price and Karen Black. Some actors, such as Béla Lugosi and Pam Grier, worked in B movies for most of their careers.