To calculate it, subtract last year's figure from this year's and divide by last year's. If revenue in a quarter was 120 million dollars and the same quarter a year before was 100 million, YoY growth is 20 percent. The same method works for profit, users, cash flow or any other measure.
The comparison removes seasonality. A retailer always sells more in the holiday quarter, so comparing it with the previous quarter would mislead, while comparing it with the same quarter last year is fairer. That is why earnings reports lean on it.
It has blind spots. A weak year makes the next year's growth look strong, and a strong year does the opposite. It also hides what is happening inside the year, such as a sharp slowdown in recent months. Acquisitions and currency changes can distort it as well.
Use YoY alongside quarter-over-quarter changes, look at several years, and check whether the growth is organic.