These are types of films known to promote intense excitement, suspense, a high level of anticipation, ultra-heightened expectation, uncertainty, anxiety, and nerve-wracking tension. Thriller and suspense films are virtually synonymous and interchangeable categorizations, with similar characteristics and features.
If the genre is to be defined strictly, a genuine thriller is a film that rentlessly pursues a single-minded goal – to provide thrills and keep the audience cliff-hanging at the ‘edge of their seats’ as the plot builds towards a climax. The tension usually arises when the main character(s) is placed in a menacing situation or mystery, or an escape or dangerous mission from which escape seems impossible. Life itself is threatened, usually because the principal character is unsuspecting or unknowingly involved in a dangerous or potentially deadly situation. Plots of thrillers involve characters which come into conflict with each other or with outside forces – the menace is sometimes abstract or shadowy.
Another closely-related genre is the horror film genre (e.g., Halloween (1978)), also designed to elicit tension and suspense, taking the viewer through agony and fear. Suspense-thrillers come in all shapes and forms: there are murder mysteries, private eye tales, chase thrillers, women-in-danger films, courtroom and legal thrillers, erotic thrillers, surreal cult-film soap operas, and atmospheric, plot-twisting psychodramas. Thrillers keep the emphasis away from the gangster, crime, or the detective in the crime-related plot, focusing more on the suspense and danger that is generated. See also this site’s listing of AFI’s 100 Most Thrilling Films.