Pilots have three travel options if they are working for most 121 carriers.
1) Pay full price like everybody else.
2) Jumpseat (IE hitch a ride in an open seat on airlines that have reciprocal agreements)
3) non-rev.
Non rev (Non-revenue travel) is a space available seat on the aircraft for the employee and their travel dependants. Depending on the company, the non-rev benefits can vary greatly.
For example, when I worked for ACA/United Express, we had free travel for us and dependents on our flights, and free travel on all other United flights (UAL and UEX). Then things changed a bit and we had free travel on our flights (regardless of the codeshare), and had to pay a minimal fee for any other United branded flights ($15 per leg on any other Express Carrier, and a set fee plus .02 cents a mile on UAL or something to that effect.)
Free travel is something that I miss greatly now that I'm out of the airlines... My wife used to "commute" each week from IAD-BOS for graduate school, and I figure that my non-rev and jumpseat travels were worth over $150,000 in free travel over my 5 year airline career (I kept boarding stubs and I had over 450 flights during that period, including international first/business class trips for free).