Pseudo-classes get regular elements but with certain conditions when they are under a particular state, structure or validation.
Examples of pseudo-classes:
:active
:hover
:nth-child
:disabled

Pseudo-elements are those which let select logical elements to be defined which are not actually in the document element tree and may only be applied to external and document-level contexts, anyway, they only apply to style specified parts of an element or a document.
Examples of pseudo-elements:
::first-line
::first-letter
::before
::after

References:
https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes
https://developer.mozilla.org/en/docs/Web/CSS/Pseudo-elements
https://www.smashingmagazine.com/2016/05/an-ultimate-guide-to-css-pseudo-classes-and-pseudo-elements/