Statistical Arbitrage is exactly what the name suggests. In Stat Arb, you (algo) basically calculate the past/historical correlation between two stocks or two assets, and try to find dislocations or price discrepancies between them.
Here is an excellent example of Stat Arb:
A Stat Arb on AAPL and the S&P can be executed as follows:
BUY [LONG] – AAPL
SELL [SHORT] – S&P
HFTs (Algos) do this all the time.