Yep, just what 2door said. So, if I upgrade my suspension, and I get a springer front suspension, the bike should ride better. You can upgrade your exhaust from that normal exhaust to the expansion chamber, the bike should run better. Upgrade the brakes, the carb, upgrade the hardware (better nuts and bolts), upgrade the seat...
The fun thing about these bikes, is you can buy a crappy Walmart beach cruiser for 100 bucks, and put 800 bucks into it in motor, upgrades.... It's a part of the fun of building these things. And for the most part, upgrades are pretty cheap, under 50 bucks (usually)