For the most part, you're interested in the core components because these are the ones that will be damaged by excessive heat:
CPU: The main 'brain' of the computer. This is the part that will usually fluctuate in relationship with the load on your computer. Both sensors are measuring the same thing, but from different places. It's probably a safe bet to average the two.
Northbridge: This is the part of the computer that connects all the other parts together. This chip can get hot, and can (like any component) be damaged by heat. The same applies about the multiple sensors: they're all measuring the same thing; you can average them.
Disk: This is the hard drive, where all your files are stored. Heat can have a significant impact on the health of your drive, so this is definitely worth taking a look at.
In general, I recommend looking up the manufacturer's specs for the component in question (all components are different) so you know what the acceptable temperature range is for it. Don't be worried unless you exceed that range.
All that said, Macs are pretty smart about managing heat. If it really gets too hot, it will turn off automatically. That doesn't mean that you can't damage your computer with heat at all, but it does mean that you generally shouldn't have to worry too much about heat issues.
