Server - includes storage and may or may not also provide a digital or analog audio output. (Network Attached Storage, Aurender N100, Cocktail Audio etc...)
Streamer - No built in storage, only receives audio streams from sources like a NAS, DLNA server, Spotify, Tidal, Internet Radio services or from a cast. Devices include premium options from Sonore, Aurender, Auralic or cheaper options from China.
Simplest would be getting a Chromecast and using your phone as a source. Cheap options also include getting a Raspberry Pi and some form of storage (microSD, USB Drive, external HDD) but that may not be the easiest option unless you are familiar with those devices.