Modem ring detect - detects when the phone rings. I believe it can be used to turn on a computer that is in hibernation so that it can answer the call.
Power-on password - enables a password that you must supply an answer for as soon as you power on the computer. If you don't answer the password you can't get past POST (power on self test.... which is what you see when it goes through checking hard drives, ram, floppy, cpu etc)
L2 Cache - Processors must have internal memroy to run. From fastest to slowest they are registers, l1 cache, l2 cache. I heard something once that Intel was going to place several megs of l3 cache on their processors.... but thats a waste if you ask me unless you can fully reserve that for the kernal (the part of the OS that is loaded in memory at all times). Outside of the processor you have much slower RAM. Don't get me wrong, RAM is sort of fast, but NO WHERE NEAR as fast as l2, l1, or registers.
Processors have very few (relatively) registers, a small l1 cache (a few kilobytes) and a larger l2 cache (a few hundred kilobytes). The RAM can be several gigs. Notice that as the capacity increases, speeds decrease.
The battery on the board keeps the system clock running so when you unplug your computer you don't have to reset your clock. It also keeps other things saved such as what your hard drive configurations are so it doesn't have to redetect them. The AC plug only powers the big stuff like your monitor and power supply in the PC (which gives power to the processor, hard drives, floopy drives, USB ports etc etc etc and also keeps the system board battery charged)
Bus/Core ratio? Desktop processor cores run at what... 3.3v? Laptops are 1.8v? Don't quote me on that.