How different between preposition and adverb

The easiest way to distinguish them (and by no means the most complete way) is to see that an adverb is a modifier. That means that it comments or restricts the word it modifies.

Today was very hot. [It wasn't just hot. It was very hot. A different kind of hot].

An adverb can do this to another adverb, a verb, or an adjective.

A preposition is an entirely different creature. It usually is about location.

The book was placed carelessly on the table.

The book was on the table. Where was it? On the table.

Prepositions do other things but what I've written get's you started.