On Mac OSX Yosemite why does my computer download 500MB every day whether I am using it or not?

nsurlsessiond deals with iCloud & also Spotlight.

• for Spotlight - Prefs > Spotlight > Search Results & switch off Spotlight Suggestions & Bing Web Searches.

• iCloud, some suggestions…

  • Disable iCloud Photo Library, Photo Stream & iCloud Photo Sharing from
    System prefs > iCloud - Photos [or equivalent, I can't remember the precise layout before Photos app]

  • If you have an iPhone, the same settings are in Settings > iCloud > Photos

nsurlsessionhd can be disabled with

sudo launchctl unload /System/Library/LaunchDaemons/com.apple.nsurlsessiond.plist

BUT you run the risk of some Apple (and maybe non-Apple) apps not being able to connect to the internet, specifically App Store, iTunes, software updater, etc. If that is the case re-enable by replacing unload with load in the command above.