it is just a pin that can be input or output depending on configuration
you can even swtich between I/O operations on fly
most of general I/O pins on MCU are universal usage, but there are exceptions.
if you are talking about TRW-2.4G module it requires 5 pins from MCU side:
3 outputs, 1 input and 1 I/O (data pin)
when data sent to radio module DATA pin works as output
when radio module send something back to MCU it must be input
if you are talking about SMIRF module it DOES use USART interface and all low-level communications with radio done in on-board controller
By the way, if you not done this yet - I suggest you to learn some very basic things with controller usage and programming (lighting LEDs, working with buttons etc) before you try something serious. I mean radio module interfacing, communication protocols and data processing is quite advanced topic!
I been working with MCU for quite some time now, but this is the first time to utilize IC to IC interface, so I wanted to be sure that I am picking the right components before starting anything.