The responses in this thread suck.
There are multiple ways to measure a fish. The most common and standard are listed below:
SL is measured snout to last vertebra
TL is measured snout to end of caudal fin (typically pinching the caudal fin together by pushing on the top and bottom of the fin)
FL is measured snout to end of middle caudal fin ray (this is how I like to measure cats because some have very long streamers on their caudal fin)