How to measure change in time series?

background

I have taken two basic econometrics classes at university and I am familiar with R.

data

weekly data about Apple app store ranking of the top apps listed (also of the sub categories such as games, weather..)

for example:

Angry Birds: week1 #15, week2 #13, week3 #1, week4 #5.....
Weather Pro: week1....


thoughts

I am thinking about trying to estimate a parameter of "turbulence or change" for each category and compare them to each other.

question

How to measure change in time series?

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