As far as I know the Slingbox is used to send programs received over your own satellite system at home, via the internet, to where you are on your travels.
Media streamers OTOH connect your old-fashioned* TV set to the internet via cable or wifi and enable you to watch anything** available on the net, on your TV.
* Old fashioned, because I understand that since I bought our flat screens a few years ago, we have progressed to TVs which are wifi 'enabled' and from there to TVs which have wifi with built-in media streamers.
** Anything that is which is not blocked for foreign consumption, such as the BBC iplayer. This is why one might need a virtual private network (VPN) which conceals your physical location.