One of the problems with tap water is that many municipalities use chloramines to disinfect the water. These chloramines can create a medicinal taste in the beer. Filtering with a carbon block filter is one way to remove the chloramines from the water.

This is the filter that I use:

It is a standard whole house filter housing that I attached some pvc pipe to the output, a garden hose quick connect fitting to the input, and some aluminum hooks. I use this filter. They claim that if you run the water through slowly (less than 1 gallon per minute) it will remove the chloramines. After using it, I take the filter out of the housing and allow it to dry before storing it.