Every once in a while, players find bugs or glitches, game systems, rates, hit boxes, level design, etc. to use to their advantage in a manner not intended by the game's designers. While it isn’t set in stone what does or does not qualify as an exploit, the aforementioned things mostly are.
Whether or not an exploit is considered to be cheating is entirely up to whether or not it gives the player using it an unfair advantage against others.
Let us take League of Legends for example. In the past, there have been several instances of players discovering ‘loss prevention’ exploits. Those players were able to get away with losing multiple matches without any decrease in their rank or LP.
However, seeing as how Riot Games is such a huge name in the industry, they have always patched those bugs or glitches as quickly as possible and, in some cases, even punished players who abused it with suspensions.