This ratio refers to the Front Side Bus (FSB) and the Memory Bus. They both use the PCI system bus of 33Mhz x a multiplier.
For example a ratio of 1:1 simply means you are running the FSB and Memory at the same speed, whatever that might be.
100MHz FSB: 100MHz memory = 1:1
133MHz FSB: 133MHz memory = 1:1
166MHz FSB: 166MHz memory = 1:1
A ratio of 3:4 means you are running your FSB at 33MHz x 3, which equals approximately 100MHz, and your memory at 33MHz x 4 = 133MHz.
3:4 is just an easier way of saying 100MHz:133MHz, provided you know what they are talking about.
The whole reason different ratios exist is so you can run your memory at a different speed from your CPU for improved performance or increased compatibility.
You will also see 4:5, for 133MHz FSB and 166MHz memory, and possibly 5:6 as well.