Zlib is the standard (de)compression library. Many programs rely on it.
 Typical errors
If your distribution did not install zlib for you during installation (CentOS, etc) then you may likely run into compile errors such as:
pp g++ -o tracker_users.o -pthread -O2 -w -I. -c tracker_users.cpp g++ -o util.o -pthread -O2 -w -I. -c util.cpp g++ -o ./bnbt util.o -pthread -Lzlib/ -lz /usr/bin/ld: cannot find -lz collect2: ld returned 1 exit status make: *** [bnbt] Error 1
The solution is to install zlib.
yum install zlib-devel