Under the IRS rules the custodial parent is the parent with whom the child lived for the greater part of the year. The other parent is the noncustodial parent.
The custodial parent may release the claim to the noncustodial parent by signing the form 8332 - http://www.irs.gov/pub/irs-pdf/f8332.pdf - should be filed with the noncustodial parent's tax return. The non-custodial parent should obtain that form signed by the custodian parent for specified years.
Please refer to the IRS publication 504 - http://www.irs.gov/pub/irs-pdf/p504.pdf
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