S2 (Style System 2) is the follow-up to LiveJournal's original templating engine, now referred to as "S1". S1 allowed people who knew HTML to somewhat easily re-style their journal but had a number of shortcomings:
The four views (recent ("lastn"), friends, calendar, and day) all had to be created independently and couldn't share any code. So if you wanted to make a new style you had to do it four times. The tediousness of this made us reluctant to add any more views to the system (such as a month view or single item/comments view).