Title sequence is that part of a movie (or TV show) wherein the title and key cast and production members are listed, often incorporating creative visuals and compelling music. Most title sequences aim to set the mood of a film. Some title sequences come before or after a prologue, while some serve as the prologue itself, giving you a glimpse of what the film is about.
It may include a simple series of drawings (e.g., The Spirit of the Beehive), live action (Lord of War), animation (Pink Panther, Catch Me If You Can, etc.), a montage (e.g., Persona), or a combination of live action and animation (e.g., Juno). Sometimes, a simple Jack-o'-lantern on the corner and some creepy music will do (case in point: Halloween). Some movies choose not to have a title sequence at all and just display the title without music (e.g., Citizen Kane).
Title sequences have evolved over the course of cinema's history. From the deep South ambiance of Gone with the Wind to the iconic James Bond title sequences to the multilingual title sequence of Enter the Void, most title sequences have been effective in immediately grabbing the viewer's attention.