A bicycle-component bearing that is self contained and pressed in place. It's designed to be easier to replace when worn out. Sealed cartridge bearings have covers to keep dirt and grit from getting inside and contaminating the bearings and grease inside.
Pedaling narrow trails requires special skills that you can develop and improve during every ride. Here's what IMBA recommends:
Start by keeping your arms flexed, not locked. On rocky or rutted trails, try to follow the path of least resistance. Imagine where you want your bike to go, and don't focus on the obstacles you want to avoid (if you do, you'll probably hit them). Learn to use both your front and rear brakes. On steep climbs, lower your elbows and drop your upper body closer to your bike's top tube. This helps you maintain tire traction. Finally, hold the handlebars firmly but not with a death grip as too much squeezing will tire you out and increase steering mistakes.