What is actually grub in dirt?

A grub in dirt is usually a larva form of a beetle, called a chrysalis or pupa. This is the stage in a beetle's life when it goes from a grub to an adult.

Life Cycle-

Egg - larva (or grub) - pupa (chrysalis) - adult beetle.

Some beetles are different. Some pupa can be a cone shape (which is what most people would think looks similar to a butterfly cocoon), or even the shape the the adult itself.

I hope I answered it right. Forgive me if I'm wrong, I'm not exactly a biologist. This is what I learned in school last semester.