You can delete individual movies and shows by unselecting the check box next to each title. If a movie or show leaves the Netflix service, all downloads for it immediately expire. Downloads can be renewed, but some have a limit on how many times per year. Each of the downloads has a different expiration date based on its content license - some expire as early as 48 hours after you first start watching. Netflix allows you to keep a maximum of 100 downloads on as many devices as are included in your subscription plan.
On Android devices, you can download an entire season of a show by tapping the separate Download Season button next to the My List, Rate and Share links near the top of a movie or show listing. TV shows can be downloaded individually by clicking the download icon next to each episode. On Android, Netflix lets you download entire seasons of TV shows.