Sky Glass is a TV that has a Sky box built in. With sky Glass, you have to buy the TV and a Sky subscription.
Sky Stream is a small streaming box that you plug into any TV via the HDMI cable. With Sky Stream, you only pay for the Sky subscription.
Another major difference is that Sky Stream does not have a hard drive and therefore you cannot record programmes onto the box. Instead, you add your programmes to a playlist and you can Stream them at any time.
The only caveat is that if the streaming service eventually removes the content it will not be available.