To perform or display a work “publicly” means—
(1) to perform or display it at a place open to the public or at any place where a substantial number of persons outside of a normal circle of a family and its social acquaintances is gathered; or
(2) to transmit or otherwise communicate a performance or display of the work to a place specified by clause (1) or to the public, by means of any device or process, whether the members of the public capable of receiving the performance or display receive it in the same place or in separate places and at the same time or at different times.
Most movies are intended for personal, private viewing only. If you're watching a movie in a group with your family and friends in your home, there is no issue under copyright law. Similarly, if you rent out a space for a private party and screen a movie for family and friends, and the party is exclusive to your guests and closed to the general public, there will not be a copyright law violation. However, if your event was open to members of the public outside your small group, the movie showing might be considered a “public performance."
If the showing is considered public, you are restricted by copyright law. Typically, you can't legally show a movie to the public unless you obtain public performance license from the copyright owner. You can obtain a public performance license by either contacting the copyright holder directly or by licensing from entities set up specifically for the purposes of licensing movies, such as Swank Motion Pictures or the Motion Picture Licensing Corporation.