A simple example to make a library (rather than an executable, which is the default). TEMPLATE variable specifies type of the project you are making. lib option allows makefile to build a library.
library.pro
HEADERS += library.h
SOURCES += library.cpp
TEMPLATE = lib
# By default, qmake will make a shared library. Uncomment to make the library
# static.
# CONFIG += staticlib
# By default, TARGET is the same as the directory, so it will make
# liblibrary.so or liblibrary.a (in linux). Uncomment to override.
# TARGET = target
When you are building a library, you can add options dll (default), staticlib or plugin to CONFIG.