There are a couple of different hardware interfaces built into the ATMega168 chips, logic-level serial (UART), SPI, And I2C (which for copyright reasons Atmel calls TWI). Serial is by far the easiest to use, and there’s a nice thread on serial talking between an Orangutan and a Baby Orangutan here. It’s somewhat long, but if you skim it you should find the parts you need. Keep in mind that the code developed in this thread is for one way communication from a Baby to an Orangutan, but you can swap the codes (not the LCD part of course), or blend the codes together for two-way communication (either way, make sure you keep the correct F_CPU clock speed definition with the correct device).
You can also implement just about any other kind of interface in software.
-Adam