I hate being so computer dumb, at least I am still good with the chicks... (not really chicks, ladies over 65 think I am a "fine young man", I will take any compliment...)
I know what RAM is, and what it does, but the other numbers are well, other numbers...
DDR is just SDRAM - DDR stand for Double Data Rate - DDR2 is the newer version which clocks internally at twice the bus rate - They now produce DDR3 ram and some graphics cards are using 4+.
800mhz is the frequency of the bus that this supports - This is overclocking memory so can go higher.
4 x 1 GB - ie. 4 sticks each 1GB
Low latency is about the number of clock cycles the controller has to wait before reading / writing to the memory - there are a number of timing items here but you get the basics. This is low latency i.e. quicker than normal.
FYI It is rated to 2.3v but will boot happily at 1.8v with stock timings - My timings are tighter than stock so I run at 2.0v
Basically all this is to make sure you buy the right stuff for your PC... your motherboard will dictate what memory you can use.