April Clones are clones of April, introduced in The Kraang Conspiracy. They need the real April's DNA to be complete. Most of the April clones turn into black slime once killed. Reject clones are mostly deformed, disgusting prototypes like April Derp, for instance.
One April clone easily fooled the Turtles and trapped them in a detention cell. She proceeded to attack them. Donnie, realizing the April was a fake, manages to defeat her, turning her into black slime.
Soon the Turtles figured out that there are many April clones settling in mutagen. The Kraang soon finds the turtles and sets a rejected April clone (soon named April Derp) free. April Derp attacks the Turtles while the real April's DNA was about to be extracted. April released a large energy blast, killing April Derp. Not long after, the encased April clones were set free, and attempted to fool the Turtles by claiming to be the real April. April identified herself by pulling out her tessen, and proceeding to kill two of the clones. Realizing that she is the real April, the Turtles pulled out their weapons and kill the remaining April Clones.