makes the bike go straight, or turn, or go straight after a turn.... in all seriousness, CC does an insane job making silent products that usually go unnoticed, as they should. Simple, light, smooth. Great value too.
The 40 series is a solid headset. Affordable, long lasting, and available in pretty much any conceivable configuration. Only thing they're lacking is bling factor, and considering the price, that's not a big deal.
The 40 Series is a workhorse of a bearing. It has a great price and will work well. If you are building up a bike that will be ridden a ton, I would suggest going with the 110 Series for longer lifespan.
Pair this with a zs-44 top, and you've got a slick, low profile headset for frames that will accept the two together. Light weight, painless install, and no creaks, cheaper than the 110, cheaper than a king, comes with a 40 year warranty. http://www.competitivecyclist.com/cane-creek-40-series-zs-44-tall-top-headset-oe#wall-solicitations