Speed
The numbers are in MHz, and represent the frequency of the clock signal at which the RAM operates (x2 for DDR RAM, so DDR2-800 is running at 400MHz). The DDR means "Double Data Rate" which means it transfers data on both the rising AND falling edges of the signal (instead of just signal on vs. off). So, for example, DDR gives you the effect of 800MHz while actually still only being at 400MHz. DDR2 and DDR3 are superseding versions of the DDR spec. (ie: DDR3 is "double data rate type three").
Timing
Memory timings (or RAM timings) refer collectively to a set of four numerical parameters called CL, tRCD, tRP, and tRAS, commonly represented as a series of four numbers separated with dashes, in that respective order (e.g. 5-5-5-15). However, it is not unusual for tRAS to be omitted, or for a fifth value, the Command rate, to be added on (from Wikipedia).
CL (CAS Latency)
The CAS latency is the delay, in clock cycles, between sending a READ command and the moment the first piece of data is available on the outputs.
LRCD
Row Address to Column Address Delay - tRCD is the number of clock cycles taken between the issuing of the active command and the read/write command. In this time the internal row signal settles enough for the charge sensor to amplify it.
tRP
Row Pre-charge Time - tRP is the number of clock cycles taken between the issuing of the pre-charge command and the active command. In this time the sense amps charge and the bank is activated.
tRAS
Row Active Time - tRAS is the number of clock cycles taken between a bank active command and issuing the pre-charge command.
See here for more info on these and other RAM timing elements.
Voltage
The listed voltage is the minimum/recommended voltage required to power the RAM module. Not enough and it can't power the module, too much and you can damage the various chips on the module.
Multi-Channel Kits
These 'kits' are simply multiple single, similar (identical as possible) RAM modules packaged together. The intention (these days) is for them to be used in motherboards that have dual and triple (etc.) RAM channel capabilities. IE: since you need 2 sticks to do dual channel, and that became standard/regular for new systems a while back (before triple channel, quad, etc.), the memory manufactures started marketing their existing 'kits' as 'multi-channel kits'.
Previously the kits were sold mainly to give a bit of a break on price when buying multiple modules (ie: Two 1GB modules in a '2GB kit' is cheaper than buying two individual 1GB modules of the same model).
DDR,DDR2 and DDR3 memories follow the DDRxxx/PCyyyy classification.
xxx indicates the maximum clock speed the memory chips support.eg DDR400 memories work at 400 MHz at the most,DDR2-800 can work up to 800 MHz and DDR3-1333 can work up to 1,333MHz.Though this is not thr real clock speed of the memory.The real clock of DDR,DDR2, and DDR3 memories if half of the labeled clock speed.Therefore DDR400 memories work at 200 MHz,DDR2-800 memories at 400 MHZ.
yyyy number indicates the maximum transfer rate that the memory reaches in MB/s.
The bus clock rate is the actual speed of ur FSB(The FSB connects the processor (CPU) in your computer to the system memory)
Best Answer
Voltage, Latency and timings are not important factors.
... Well, Voltage is very important - but if you get standard grade desktop commercial components, they should all meet the standard. It is just some overclocked or special server memory uses different voltage, however they are usually more expensive and warn you.
Latency and timings, there are rare instances where it is not possible to have differences - usually some chipsets/motherboards prevent this, however, in most instances you will just get a warning when you start the machine and it will operate at a low speed (or even a penalty speed where it is slightly lower than the lowest - only seen this once on a American Megatrends BIOS years ago).
As it is part of your question - it shouldn't cause any damage to the machine. The machine is designed for the memory socket. At worst you will get a beep warning and/or the memory will run slow. I cannot imagine any irreversible damage. Even if you had memory requesting higher voltage, I could imagine the memory stick failing before the motherboard - but don't test it or quote me on that!
As for buying, my guide will always be to try and buy a matching / identical second stick as this enables dual (or higher) channel memory support, if your motherboard supports it. This mode can (under some circumstances) dramatically increase memory performance.