/testfor command has many uses. For example, you might want to testfor a player with a score of at least 3 on the red team. To do this, you would use the following command:
In this command, you testfor the nearest player with a score, on the objective "objname", of 3. This player would have to be on the red team. If you attach this command block to a clock, and put a comparator on the side of it, whenever it finds a player, it will output a redstone signal.
Click HERE for a complete list of selectors that you can use within the square brackets.