Non-standard LEGO themes
There are huge number of different LEGO set themes these days. A lot more than the original three introduces back in 1978 (city/town, castle and space). Most of those themes use (mostly) standard LEGO bricks, but there are few that have their own distinctive pieces.
The most well known such themes are Dublo- and Technic sets. Former featuring big, double sized bricks and later using pins instead of studs for primary connectors between bricks.