Oh no, they expire.
Basically, season tickets tend to come in two flavors...
For teams with rather low demand (Like, say, southern NHL teams, and most (all?) baseball teams), the season ticket represents a significant discount over single game tickets, and generally are relatively easy to acquire - I had Carolina Hurricanes tickets for a few years, and by the time you add up the steep discount on the tickets themselves, flat rate parking pass (Not included, but again, a steep discount), and not paying any Ticketmaster/"Convienence" fees, it cost me less than half (on a per game basis) what buying single game tickets would cost.
At the other end of the spectrum, you have high-demand teams that sell out 99.9% of games, like most Canadian NHL teams, and many NFL teams. There, practically all seats are held by season ticket holders (whose annual season ticket price is usually the same as, or sometimes even HIGHER than (hypothetical, generally not available) single game ticket pricing), and where the "rights" to a season ticket can trade for lots of $$$ (thousands) on the secondary market.... you can't just buy tickets, you have to buy an existing ticket holder's account, essentially. A bit like taxi medallions in NYC.