Netfinder simply shows you the download speeds you are getting right now.
Netfinder does (or tries to do) an actual download continuously at short intervals. By doing this (rather than lengthier speed tests), you can follow the signal and find where its strongest.
Good for campers or other situations where internet reception is spotty.
NETFINDER USES REAL DATA, so keeping it open for long periods of time will consume large amounts of your data plan.