MAT is minimum alternative tax section 115JA was replaced by
section 115JB for assessment years 2001-02 onwards. As per
this section, your normal tax payable is to be compared with
7.5% of your book profits and you are required to pay the
higher of the two. From assessment year 2007-2008 onwards
your normal tax payable will be compared with 10% of book
profits are your required to pay the higher of the two.
Credit for MAT paid can be carried forward under section
115JAA for 5 years.

Now mat credit is avilable for 7 years .
MAT is minimum alternative tax incase compnay have no tax 
liability as per incometax act ,so compnay pay mat @10%.
some of items addedback & deducted for calculating mat .