Legendary levels were created several years ago. Before their introduction, the highest traditional level you could reach was level 70. Legendary levels go up to 800 and have a separate place in the rankings. If a player has obtained a legendary level, when you select them they will be shown as a level 70 player, with their legendary level underneath that, or to the right on the world map. Example: Level:70, Legendary Level: 434. Or when taking a look at their Player InfoBox, it will look like this:

Legendary Lvl Image

Player InfoBox, with legendary level.