IRS debt that comes due in the three years prior to filing your bankruptcy will be given priority status and must be paid in full through your case. A tax comes due in April of the year following the date it was incurred. For example, your 2012 taxes came due in April 2013. Taxes that are older than three years will typically be treated as unsecured debt.
However, there are caveats to these rules. For example, if your taxes were filed late or were assessed by the IRS within 240 days of filing the bankruptcy then they still may be afforded priority status. Federal tax liens are also an exception, and must be paid through the case with interest in most circumstances. Once you file the bankruptcy, the interest and penalties will stop. This is a big benefit of the Chapter 13 case for those with recent tax debt.
Keep in mind that the IRS can offset your tax debt by intercepting your federal tax refunds during the first year that you are in bankruptcy. If tax refunds are intercepted during your case, then the IRS will have to amend their proof of claim and give you credit for the amount intercepted.
Tax problems are often complicated and should be disclosed to your attorney for the best information as to how the bankruptcy can help.