Water Filters
While on a camping trip, water is one of your most precious items. Sometimes, however, mother nature doesn't want to cooperate and provide a source of fresh, clean water to drink. That is when your crew will need a water filter or a water purifier.
"What's the differences between a filter and a purifier," You ask? A filter removes microscopic
organisms by pushing the water through a screen or filter which only lets the water through and keeps the bad guys on the other side. The holes in these filters are extremely small and are measured in microns. A two micron hole measures just two millionths of a meter, or 2/1,000,000 meters (if you never got past high school chemistry just know that it's pretty darn small). A purifier works the same as a filter, but permits nothing to pass through except pure water. Filtered water can also be made pure by adding a hint of iodine, chlorine, or the like to the water to kill any living organisms that may have gotten through. Filtered water can also be boiled for one minute or more to kill bacteria and viruses before you use it
Should I Get One?
So now you're asking, "Should I get a filter or a purifier"? The answer is really, it doesn't 
matter too much. Each will provide you with plenty of protection. But if you are going to go on a long trip (a week or more) where you might be near water sources known to be contaminated with Giardia and Cryptosporidium (two particularly nasty bugs), you'd likely choose to take a purifier or have a chemical additive for your filtered water. This just adds a little safety factor because nothing spoils a trip faster than intestinal problems.
When buying a filter, consider the pump rate, or how fast it pushes water through the filter. 
Some are faster than others. Be sure you can replace the filter cartridge too. The average life span for the Pur Hiker or a Sweetwater Guardian (two leaders in the filter market) filter cartridge is about 100 gallons. If possible, clean your cartridge occasionally to extend its service life.
Other Options
Now if you are still a little wary about filters, you can always use plain 'ol iodine tablets. 
Note: If you are using iodine tablets or other purifier that adds iodine to the filtered water, and you have thyroid problems, consult your doctor. Iodine can sometimes have adverse health affects.