what are the differences between sockets(478,754,775,etc.)
and
which are better/worse?

The various sockets that exist today are the foundations for the recent generations of CPU's. Here's a quick run-down...

Intel

Socket 478 - Slightly older socket that is home to the Pentium4 (Willamette, Northwood A/B/C, Prescott), the Pentium4 EE (Gallatin), and the Celeron (Willamette-128, Northwood-128, Prescott-256).

Socket T (LGA 775) - Latest socket that is home to the Pentium4 (Prescott, Prescott-2M), the Pentium4 EE (Gallatin, Prescott-2M-EE), the Celeron D (Prescott-256), and the Pentium D (Smithfield).

AMD

Socket A (462) - Older socket that is home to the AthlonXP (Palomino, Thoroughbred A/B, Thorton, Barton), Sempron (Thoroughbred B, Thorton, Barton), and many older AMD CPU's.

Socket 754 - Latest lower-end socket that is home to the Athlon64 (Clawhammer, Newcastle), and the Sempron (Paris, Palermo).

Socket 939 - Latest high-end socket that is home to the Athlon64 (Newcastle, Winchester, Clawhammer, Venice), Athlon64 FX (Sledgehammer, Clawhammer), and the Athlon64-X2 (Toledo).

As to which one is better/worse, well that's not exactly an applicable question. Which ever CPU you have or want or are working with is always the best slot/socket for that CPU! :p In addition to these more recent CPU interfaces, there are many more, including:

Socket 1/2/3/4 - 486-era CPU's. Socket 5 - early Pentium-era CPU's. Socket 7 - Pentium, K6 series CPU's. Slot 1 - Pentium II, Pentium III CPU's. Socket 370 - Pentium III series CPU's. Slot A - early Athlon CPU's. Socket 8 - Pentium Pro CPU's. Slot 2 - Pentium II/III-based Xeon CPU's. Socket 640 - Pentium4-based Xeon CPU's. Socket 940 - Opteron, early Athlon64 FX CPU's. And many many more.