The GROUP BY clause groups rows on the basis of similarities between them. You could, for example, group all the rows in the PLAYERS table on the basis of the place of residence; the result would be one group of players per town. From there you could query how many players there are in each group. The question that is actually answered is then: How many players live in each town? Other examples follow: How many matches have been played per team, and how much has been incurred in penalties per player? In short, the GROUP BY clause is frequently used to formulate questions based on the word per.
By adding aggregation functions, such as COUNT and SUM, to a select block with the use of a GROUP BY clause, data can be aggregated. These functions owe their name to this. Aggregation means that you ask not for the individual values, but for summations, averages, frequencies, and subtotals.