A Roku is a device to watch streaming content(Hulu, Netflix, etc) and music streaming apps(Spotify, Pandora, etc) and other video streaming stuff
My family and I used one and loved it - then we got a home theater in a box system with an internet connected blue-ray player that streams all that stuff. No longer use the roku because the home theater is connected... the roku was great, though.
My partner has a roku that we use in the living room. I'm unsure of what one it is though. It is a nice thing to have, especially when you don't have cable.
I know someone who has a Roku player. It is actually really cool. It is a Roku 1. I don't really see the point of getting a Roku 2,3, or 4 for a higher price with not much difference. I've actually consider getting one.. but I probably won't though. XD