Proper matching subfloor types / composition to the type of flooring you wish to install is very important.

Particleboard
Particleboard is made of wood fibers that have been glued together under pressure.  It has a very smooth, hard finish, but it is very vulnerable to moisture. 
It is not recommended as a sub-floor material for solid or engineered flooring that is to be nailed in place, as the nails or staples will pull out or become loose.

--------------------------------------------------------------------

OSB
OSB refers to Orientated Strand Board, and it is widely used in residential and commercial projects.  OSB is a very strong material that is made of wood pieces that are bound together.  It is impact and fire resistant and works well as a sub-floor material for accepting a "nailed" installation.

--------------------------------------------------------------------

Plywood
Plywood may be the most widely used material for sub-floors.  As with OSB, it works well with flooring that is to be nailed in place.

--------------------------------------------------------------------

Concrete
Concrete sub-floors are found in basements and in houses without crawl spaces.  Concrete is susceptible to holding moisture and it is not advised to install solid wood flooring over concrete*.  Engineered flooring may be glued to, and Laminate flooring floated over concrete sub-floors.

* It is possible to install solid wood flooring over concrete with an adhesive sheet type material (i.e. Elastilon).  Ogle's Hardwood Flooring has considerable experience using this material.
--------------------------------------------------------------------

Gypcrete
Gypcrete is a light-weight, self-leveling, cement type mixture that may be used as a leveling compound, or in some instances as a noise deadening layer.  
Gypcrete is applied over other sub-floor materials (see above types).
The condition of any previously applied gypcrete has significant bearing on types of flooring that can be installed and the installation methods that can be used.


Note:
Installation over a Crawlspace
Please keep in mind that the direction in which your hardwood flooring boards are installed is dependant upon the direction(s) in which your floor joists are running.  Floors should be installed 'perpendicular' to floor joists.