Smaz is a simple compression library suitable for compressing very short strings. General purpose compression libraries will build the state needed for compressing data dynamically, in order to be able to compress every kind of data. This is a very good idea, but not for a specific problem: compressing small strings will not work.
Smaz instead is not good for compressing general purpose data, but can compress text by 40-50% in the average case (works better with English), and is able to perform a bit of compression for HTML and urls as well. The important point is that Smaz is able to compress even strings of two or three bytes!
For example the string "the" is compressed into a single byte.
To compare this with other libraries, think that like zlib will usually not be able to compress text shorter than 100 bytes.