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Lifters, tappets, same thing. In the Box they are a bucket filled with oil sitting on the end of the valve stem. The cam lobe bumps on the tappet which then transfers the motion to the valve.
The function of the oil filled tappet is to be able to expand or collapse slightly to allow for wear, temperature, etc., thus not needing to be adjusted like a "solid lifter" engine such as the older 911's, VW Beetle, and some of the small block Chevys, among a lot of others.
For many years the big drawback to the hydraulic lifter was that they would "pump up" at high rpm and leave a valve hanging open to the detriment of an oncoming piston. More modern designs have addressed this problem, so you seldom see a solid lifter engine in a street car anymore.
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