RO is the most convenient and effective method of water filtration. It filters water by squeezing water through a semi-permeable membrane, which is rated at 0.0001 micron (equals to 0.00000004 inch!). This is the technology used to make bottled water, it is also the only technology capable of desalinating sea water, making it into drinking water.
Non-RO water filters typically use a single activated carbon cartridge to treat water. They are much less effective, and the pore size on these filter media are much bigger, generally 0.5 - 10 micron. They can filter out coarse particles, sediments and elements only up to their micron rating. Anything finer and most dissolved substances cannot be filtered out. As a result, water is far less clean and safe compared to reverse osmosis filtration.