Probably both. I could be wrong, but less calories probably means less alcohol and vice versa. The term "light" when referring to beer certainly has multiple meanings anyway.
Yes, alcohol = calories. Dead calories at that. Once you start moving away from stouts and heavy ales the large percentage of calories come from the alcohol content, with lagers it's about 80%+, with spirits you're looking around 95%.
In the UK the term light is basically only used for the American/Canadian branded light beers (Bud, Miller, Coors etc). In terms of colour we say pale.