It gets tricky when you first meet it.
There are three things you need to consider, and you will get used it after a while:
RELIABILITY - Do you get reliable results? i.e. minimal stop/start time errors, or minimal measurement errors etc...
ACCURACY - How close is your value from the experiment to the accepted value? For example I got a 9.6 ms^2 in physics for a practical which was trying to determine the acceleration due to gravity, which is 9.8ms^2. So therefore that experiment was quite accurate.
VALIDITY - Is your experiment correct/appropriate for what your trying to find? Once again, I'll refer to the physics experiment because it's easier to explain. For example, if I am trying to find the acceleration due to gravity, executing an experiment to find the force on an object due to gravity is NOT VALID. YOU ARE LOOKING FOR ACCELERATION, NOT FORCE.
Hope this helps,