The PostgreSQL 9.0 Reference Manual  Volume 1A  SQL Language Reference
by The PostgreSQL Global Development Group Paperback (6"x9"), 454 pages ISBN 9781906966041 RRP £14.95 ($19.95) Sales of this book support the PostgreSQL project! Get a printed copy>>> 
6.8 Geometric Types
Geometric data types represent twodimensional spatial objects. Table 620 shows the geometric types available in PostgreSQL. The most fundamental type, the point, forms the basis for all of the other types.
Name  Storage Size  Representation  Description

point  16 bytes  Point on a plane  (x,y)

line  32 bytes  Infinite line (not fully implemented)  ((x1,y1),(x2,y2))

lseg  32 bytes  Finite line segment  ((x1,y1),(x2,y2))

box  32 bytes  Rectangular box  ((x1,y1),(x2,y2))

path  16+16n bytes  Closed path (similar to polygon)  ((x1,y1),...)

path  16+16n bytes  Open path  [(x1,y1),...]

polygon  40+16n bytes  Polygon (similar to closed path)  ((x1,y1),...)

circle  24 bytes  Circle  <(x,y),r> (center point and radius) 
A rich set of functions and operators is available to perform various geometric operations such as scaling, translation, rotation, and determining intersections. They are explained in section 7.11 Geometric Functions and Operators.
ISBN 9781906966041  The PostgreSQL 9.0 Reference Manual  Volume 1A  SQL Language Reference  See the print edition 