Good ones are effective and convenient because they don't require delivery as is the case with bottled water. Here is a description of a few of the methods. These includes lead, chlorine, pesticides, asbestos, cryptosporidium, giardia, and other bacteria contaminants. They work by passing the water through a dense carbon block which traps impurities. Unlike other carbon filters, the block's density prevents bacteria from growing and being passed into the water.
Some models also have an additional membrane that removes fluoride.
These systems run the water through a membrane that prevents the passage of contaminants. In the process the impurities and excess water are flushed out of the system. Some reverse osmosis systems, if combined with a supplemental carbon filter, can be effective at removing most organic and non-organic contaminants. These systems are not as convenient as the carbon block because they often take longer to provide water and also waste significant amounts of water. Another disadvantage is that these systems remove trace minerals that are beneficial to good health.