Slots
This is the total number of memory upgrade slots (sockets) followed by their configuration. Banks are the way a system addresses memory. A bank must be completely filled with memory modules of the same size and type in order for the system to recognize and address the memory. i.e. :
3 (3 banks of 1) This indicates that there are 3 memory slots. These are divided into 3 banks, and each bank consists of one memory slot. So you can add memory one piece at a time for the system to use.

Fill All Ram Slots Or Nothing

No, they do not all have to be the same although many here have found that the should be the same in side-by-side slots. I am currently running the configuration below. Many on this forum, as do I, also recommend that you use only Crucial or OWC RAM chips as bargain brand RAM often will not work. Slots are very useful for library calls to eliminate the 'named method dispatch' when making function calls. This is mentioned in the SWIG documentation. For high performance libraries that want to reduce function overhead for commonly called functions using slots is much faster. Now this may not be directly related to the OPs question. Ok my friend, now for the real answer - it will in fact hinder you performance to fill all your slots as most motherboards today are made for either dual or tri band memory and the dual channel ONLY works if you install it in the 2 specific slots its meant to be install it in. Installng more than 2 ram modules negates the dual ram function and therefore performance loss - also dual channel has.

Fill All Ram Slots Or Not Available

4 (2 banks of 2) This indicates that there are 4 memory slots. These are divided into 2 banks, and each bank consists of two memory slots. So you must add memory two pieces at a time (they must be the same size and type of memory) in order for the system to benefit from the upgrade.

12 (3 banks of 4) This indicates that there are 12 memory slots. These are divided into 3 banks, and each bank consists of four memory slots. So you must add memory four pieces at a time (and they must be the same size and type of memory) in order for the system to benefit from the upgrade.