It deals with floating-point, arbitrary-precision decimal numbers, whereas math/big contains support for arbitrary-precision integers, rationals and binary floating point. Looking at it briefly, it looks like it uses math/big under the hood when the numbers are large, but uses an internal compact form for efficiency when numbers are small ~<= 20 decimal digits.

The key word is "decimal", this is a library for decimal floating point arithmetic. `math/big` uses binary floating point. The difference is in which floating point numbers can be represented without requiring an infinite expansion. Lot more info at http://speleotrove.com/decimal/.