There are two basic camshaft designs in OHV (overhead-valve), pushrod engines: flat-tappet and roller-tappet. Tappets are also known as lifters, and the shape of the lifter is a "giveaway" as to which one is which. Flat-tappets have a flat base, while roller-tappets have a roller wheel at their base.
In the mid 1980s, GM began using roller lifters to replace the flat tappets in some of their performance engines. Soon after that, it became a common industry standard. Today, all OHV engines have migrated to that design.
Roller lifters -- by way of the roller wheel -- are a more efficient design. They create less friction. The use of roller lifters requires that the cam lobes have a different profile/shape as well. The flat-tappet design must account for the outside edge of the lifter when rising up and down the cam lobe. Roller lifters don't have that limitation, so the lobes can lift the tappet much more quickly. This results in a more efficient and powerful valve event.
Roller cams were used long before they became commonplace on production automobile engines. There are two types: mechanical roller- and hydraulic roller-tappet camshafts. The mechanical roller cams used in off-road and racing engines require that a pre-set clearance be used for the valve lash, while hydraulic roller cams (used in production vehicles) operate with no valve lash. The oil-fed lifter design of hydraulic lifters automatically compensates for this.
