What is a Concessional Contribution?
A Concessional Contribution is made to a complying superannuation fund which in turn, becomes part of the assessable income of the fund.
The individual (or entity) making the contribution is generally allowed to claim a tax deduction for these contributions. 
What is the Concessional Contribution Cap?
Concessional Contributions made by or on behalf of an individual are subject to a cap or set limit in each financial year.
For the 2010-11 financial year the concessional contribution cap (CCC) amount for individuals under the age of 50 is $25k. The cap is indexed annually.  Note that the CCCs apply to the individual, not the fund.
As an example: if Mr Jones (aged 48) made $25k in Concessional Contributions to his particular fund of choice in the 2010-2011 financial year, he is unable to contribute any additional concessional amounts to this or any other complying super fund.
Between 1 July 2007 and 30 June 2012 an increased CCC applies to individuals 50 years+ (on the last day of the financial year.) The transitional CCC for 2010-11 is $50k. This transitional cap is not indexed.
It has been proposed that from 1 July 2012 individuals aged 50+ with total super below $500k will permanently increase to $50k. This change has not yet been made law and may well be amended when passed by parliament.
What’s included in Concessional Contributions?
 Included in the list of Concessional Contributions are:
 – Super Guarantee Contributions made by employers
– Salary sacrifice amounts; and
– Personal deductible contributions