Founded by former employees of the defunct Toronto Telegram, the Sun features a right-wing editorial bent to it. Its main competition is The National Post, which unlike the Sun, is a national (in this sense, Canadian) newspaper.
If one reads the propaganda published by the Sun, one would believe that the newspaper wars in Toronto are won, and by the Sun. In actuality, this is not the case.
As emil greer says in his writeup, the Sun's main features are ads and the "Sunshine Girl". The paper also has excellent sports coverage, but papers such as The Toronto Star are quickly gaining even there.
At first, the Sun was owned by the employees. For a while, however, they were owned by Rogers, the Canadian cable giant, before buying themselves back. Now, like emil says, they're owned by Quebecor. They recently became unionized, after deciding against doing such earlier on. Being unionized is what killed the Telegram, so here's hoping it does the same for the Sun.