depends on your motherboard and a read of the speedfan site/forums will probally tell you unless its a brand new board. If you tell us what motherboard and processor you are using, maybe someone here will know.
The issue is that speedfan is reading information from the on board monitoring chip and manufacturers can hook them up in different ways so there is not one pat answer to your question.
OK no one knows exactly for for sure what is known.
Temp 2 is related to cpu temp, best quess is that it is a thermal probe located under the cpu in the "well" in the middle of the socket. This is easy to verify as you can run some cpu stress testing software and it quickly raises temp 2. However it does not exactly correspond to the cpu core temps and reacts a bit slower, thus the conclusion its a thermister under the cpu.
Temp 1 is also a thermister somewhere on the board that is reading "system temp" we really dont quite know what exactly, its near something warm on the board but rises no where near a quickly as the cpu related ones. It is more of a general "case temp" kinda thing.
temp 3 is hooked to nothing and is completely "wacko" disregard it.