The camshaft of most assembly line produced OHV automotive engines are installed into the cylinder block with common bearings such as these:
www.valiantspares.com.au/images/categories/Stock%20Parts%2018.7.8%20014.jpg
Pressurized oil is feed into the small space between the bearing and camshaft for lubrication or the heat generated by the moving parts would weld the pieces together.
Some performance engine builders (including all NASCAR engine suppliers)will enlarge the opening of the cam bore to install oversized needle bearings in place of those above.
www.rbcbearings.com/rollerbearings/index.htm
The camshaft spins much more freely in the bore resulting in less friction and thus producing more HP.
Not only the camshaft gets this upgrade but also rocker arm asseblies, gear drives and pilot bushings. And in some applications, the crankshaft and connecting rods as well.
(Response as applies to automotive performance applications only. Common in assemply line motorcycle and small engines)