Pentium, like Celeron, is just a trademark which Intel insists on hanging on it like a childhood teddybear.
The actual pentium processor lineups are: Pentium, Pentium Pro, Pentium II, Pentium III, Pentium 4, Pentium D
These don't mean much though because Pentium 4 alone had 5 different core revisions across 4 die shrinks and spanned 3 sockets on the desktop alone (socket 423, socket 478, socket 775).
Now pentium is just a generic identifier for low end Intel processors and does not refer to a particular chain of architectures.
Core 2 processors are based off of the Core microarchitecture which itself is based off the Pentium III (P6) architecture.
Nehalem takes some features from Core 2 and some from Netburst but is an entirely new...