Roku boxes have been a big hit in the States, where they allow users to stream all sorts of content, including TV, music, games and apps for media and social networking. Doubtless there will be some bits and bobs missing from the UK release due to licensing issues - TV streaming service Hulu, for one - but with LoveFilm, iPlayer, 4od and the like all offering streaming services through games consoles already, expect the overall service to be basically the same.
Roku charges a flat fee for one of their boxes (there are currently four different models currently available in the States, costing $50, $60, $80 and $100 depending on the sort of bells and whistles, like full HD streaming, the customer wants). There's a load of free content included with that, but services like Netflix movie rentals require extra subscriptions. If the pricing stays the same for the UK launch, it will be in direct competition with services provided by Sky and its cohorts.