All Drivers are modules. Not all modules are not drivers.

Modules can be inserted at runtime. Modules/Drivers can be statically compiled along with the kernel also.

Typical module init has 

 module_init(init_fn);
 init_fn()
 {
 /* some code */
 }

The same module can be made a driver 

 module_init(init_fn);
 init_fn()
 {
 device_register(&device);
 /* some code */
 }