Seniority is where you stand relative to the other pilots on the list.
Years of service are how long you've been with the airline.
There's also a term known as adjusted longevity date which relates to how you're paid. Under some pilot contracts, if you have 5 years of service but you've been furloughed for the past two, you will be paid at the third year rate for your seat when you return.
Look, I was helpful. Mark this date on your calendar, because such events are few and far between.