What is a module in verilog

The main building block in Verilog is the module. The circuits are build by connecting modules together.
Simply speaking, a module can be considered as an IC. Please have a look at this simple adder module which can be considered as an adder IC. It takes some data and produce output. Here, in case of adder, the output is sum.

Communicating with the outside world

Modules use ports to communicate with the outside world.
The data is passed to the module through input port. The port can be a single bit line or a bit vector.

The output of the modules are present in output port. This port can be a single bit or a bit vector.
There are inout ports which are capable of communicating in both directions.

The funtionality of a large circuit is divided into modules. Then each modules are designed and tested. Finally these modules are interconnected to make the final circuit. Different modules are interconnected with wires.