BPOU stands for Basic Political Operating Unit.  What that really means though, is that a BPOU is the lowest level of political organization.  In rural areas, a BPOU may be an entire county.  However, in urban areas, the BPOU encompasses only a particular Senate District or even a single House District.

The BPOU operates as a fairly independent group.  It has it's own leadership, membership, budget, etc.  Each BPOU endorses members during election years at it's caucuses.  And during off-election years holds conventions to elect officers and other business.

However, a BPOU is also part of hierarchy.  Unless, the BPOU is a House District, it will have House Districts within it.  Each House District is representated in the state legislature by a Representative.  In addition, each BPOU encompasses quite a few precincts.  These precincts vary in size but are used to help organize votes and voting locations on Election Day.

A BPOU may contain an entire city, or only part of one.  It may contain several cities, or be part of several.  A BPOU may encompasses  an entire county, or only part of one.  It may also contain parts of several counties as well.

To make this even more confusing, the boundaries of a BPOU can change when the legislature does it's redistricting every ten years.