The engine top end covers everything above the deck: camshafts and cam gears, cylinder heads, valves, valve springs and retainers, lifters and rocker arms, head gaskets, head stud and bolt kits, and valve covers.
Top end work is where naturally aspirated European engines find their power and where boosted ones find their reliability. On an S54 or S65 a camshaft and valvetrain package moves the powerband; on an N54, EA888 or 2.5 TFSI the priority is instead clamping the head down properly, because factory torque-to-yield head bolts lose their grip as cylinder pressure climbs.
Two ordering notes. Valve springs must be matched to the camshaft's lift and the RPM you intend to use — running an aggressive cam on stock springs is how valves meet pistons. And any time the head comes off, the head gasket is a one-time part; plan on replacing it.
Go deeper: cylinder heads and head stud kits, head gaskets, cam and valve seals, and ARP head studs and engine fasteners. The rotating assembly is in engine bottom end.