Currently, the click just takes you to the relevant 7Digital page for the album. In the future, however, the companies hope to allow one-click downloads in Spotify itself, as well as functionality to buy entire playlists.
This move should further silence the doubters who claim that Spotify has no business model. On the contrary, this is now a third solid way of monetising their business, after ads and premium subscriptions. I do doubt a little how much people will use the functionality, though.
7Digital and Spotify