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").
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.
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.
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.
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.
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.
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).