From 2builders2tools wiki

Exploits are the use of cheats coded into Minecraft. They give unfair advantages to players who are using a regular/non-hacked Minecraft client. They were first developed in 2009 and were later used in multiplayer servers to gain unfair advantages. Servers usually coded anti-cheats to combat hackers and exploiters. This did not happen with 2b2t, however, because 2b2t was an anarchy server and allowed cheats. Over time, exploits developed from simple spawning things into your inventory into KillAura, BHop, Reach, and more. They are still used today. Some examples of hacked clients are Impact, Kami Blue, Future and many more..