<p>I was reading a MS Excel help article about pivotcache and wonder what they mean by <em>OLE DB and ODBC sources</em></p>

<blockquote>
  <p>...You should use the CommandText
  property instead of the SQL property,
  which now exists primarily for
  compatibility with earlier versions of
  Microsoft Excel. If you use both
  properties, the CommandText property’s
  value takes precedence.</p>
  
  <p>For <strong>OLE DB sources</strong>, the CommandType
  property describes the value of the
  CommandText property.</p>
  
  <p>For <strong>ODBC sources</strong>, the CommandText
  property functions exactly like the
  SQL property, and setting the property
  causes the data to be refreshed...</p>
</blockquote>

<p>I really appreciate your short answers.</p>

On a very basic level those are just different APIs for the different data sources (i.e. databases). OLE DB is newer and arguably better.
You can read more on both in Wikipedia:
OLE DB
ODBC
I.e. you could connect to the same database using an ODBC driver or OLE DB driver. The difference in the database behaviour in those cases is what your book refers to.