Viroids are similar to viruses however much simpler and smaller ( 1/10 the size of the smallest virus) in design. Unlike a virus they don’t have a protein coat, or any other enzymes. They are simply a single stranded circular piece of RNA.
Although they may look different to viruses, they act much in the same way. They enter a cell and use the cells machinery (RNA polymerase) to copy its self and create more Viroids.
So far, Viroids have only been found to affect plants