Suppose you have two columns of data. Doesn't matter what kind of data they contain. Let it be customers and employees. You want to find common part of these two columns.
Column A - name of your clients
Column B - your employees
Column C - common part of these tho collections of data.
Your task is to check if your employees are your clients as well.
Answer: Use such a formula:
Just copy that formula in C2 cell and drag it down.
Further reading: Vlookup